user10100: I ask this question because in pali, "to let go (nibbāna)", in this context, is known by monk. It is not just a target. The (ariya) monk thinking of nibbāna in his concentration, in this context.
I just wanna make sure that english translation is right, because when I translate that english sentence, by my terrible english, it look like "to let go " and "object" means "target", not "something is known by monk".
In the pāli: ārammaṇa means "something is known by consciousness", "something is thought by consciousness", "external sense-fields". It doesn't means target/object.

"And what is the faculty of concentration? There is the case where a monk, a disciple of the noble ones, making it his object to let go, attains concentration, attains singleness of mind. 
  http://www.accesstoinsight.org/tipitaka/sn/sn48/sn48.010.than.html


Edited:
It should to be:

"And what is the faculty of concentration? There is the case where a monk, a disciple of the noble ones, making "to let go" as his mind's focus, attains concentration, attains singleness of mind. 


user254: it look like "to let go " and "object" means "target", not "something is known by monk".
Yes, I think the English is a bit ambiguous, and it could be understood to mean either (or both).
The word "object" has many meanings, of which two are:


a person or thing to which a specified action or feeling is directed.
  "disease became the object of investigation"
synonyms: target, butt, focus, recipient, victim
  "he became the object of fierce criticism"  
a goal or purpose.
  "the Institute was opened with the object of promoting scientific study"
synonyms: purpose, objective, aim, goal, target, end, end in view, plan, object of the exercise


So an English reader might understand it as the focus of meditation (what you're attentive to), or as the purpose of meditation (what your aim or ambition is).

user-1: Let my person give an answer that might be releasing in many ways :-)

Carrying a Rock
"Letting go" actually means this: It's as if we're carrying a heavy rock. As we carry it, we feel weighed down but we don't know what to do with it, so we keep on carrying it. As soon as someone tells us to throw it away, we think, "Eh? If I throw it away, I won't have anything left." So we keep on carrying it. We aren't willing to throw it away.
Even if someone tells us, "Come on. Throw it away. It'll be good like this, and you'll benefit like that," we're still not willing to throw it away because we're afraid we won't have anything left. So we keep on carrying it until we're so thoroughly weak and tired that we can't carry it anymore. That's when we let it go.
Only when we let it go do we understand letting go. We feel at ease. And we can sense within ourselves how heavy it felt to carry the rock. But while we were carrying it, we didn't know at all how useful letting go could be.

One might fear to try it, but than it's known, possible to make it a valide direct perception again.
[Note: This is a gift of Dhamma, not meant for commercial purpose or other low wordily gains by means of trade and exchange, directed to Nibbana, to simply let go]

10100: <p>I ask this question because in pali, "to let go (nibbāna)", in this context, is known by monk. It is not just a target. The (ariya) monk thinking of nibbāna in his concentration, in this context.</p>

<p>I just wanna make sure that english translation is right, because when I translate that english sentence, by my terrible english, it look like "to let go " and "object" means "target", not "something is known by monk".</p>

<p>In the pāli: <strong>ārammaṇa</strong> means "something is known by consciousness", "something is thought by consciousness", <strong>"external sense-fields"</strong>. It doesn't means <strong>target/object</strong>.</p>

<blockquote>
  <p>"And what is the faculty of concentration? There is the case where a monk, a disciple of the noble ones, <strong>making it his object to let go</strong>, attains concentration, attains singleness of mind. 
  <a href="http://www.accesstoinsight.org/tipitaka/sn/sn48/sn48.010.than.html" rel="nofollow noreferrer">http://www.accesstoinsight.org/tipitaka/sn/sn48/sn48.010.than.html</a></p>
</blockquote>

<hr>

<p>Edited:</p>

<p>It should to be:</p>

<blockquote>
  <p>"And what is the faculty of concentration? There is the case where a monk, a disciple of the noble ones, <strong>making "to let go" as his mind's focus</strong>, attains concentration, attains singleness of mind. </p>
</blockquote>
 254: <p><strong>it look like "to let go " and "object" means "target", not "something is known by monk".</strong></p>

<p>Yes, I think the English is a bit ambiguous, and it could be understood to mean either (or both).</p>

<p>The word "object" has many meanings, <a href="https://www.google.fr/search?q=define%3Aobject" rel="nofollow noreferrer">of which two are</a>:</p>

<blockquote>
  <ul>
  <li>a person or thing to which a specified action or feeling is directed.<br>
  "disease became the object of investigation"<br>
  <em>synonyms</em>: target, butt, focus, recipient, victim
  "he became the object of fierce criticism"  </li>
  <li>a goal or purpose.<br>
  "the Institute was opened with the object of promoting scientific study"<br>
  <em>synonyms</em>: purpose, objective, aim, goal, target, end, end in view, plan, object of the exercise</li>
  </ul>
</blockquote>

<p>So an English reader might understand it as the <em>focus</em> of meditation (what you're attentive to), or as the <em>purpose</em> of meditation (what your aim or ambition is).</p>
 -1: <p>Let my person give an answer that might be releasing in many ways :-)</p>

<blockquote>
  <p><strong><a href="http://www.zugangzureinsicht.org/html/lib/thai/chah/insimpleterms_en.html" rel="nofollow noreferrer">Carrying a Rock</a></strong></p>
  
  <p>"Letting go" actually means this: It's as if we're carrying a heavy rock. As we carry it, we feel weighed down but we don't know what to do with it, so we keep on carrying it. As soon as someone tells us to throw it away, we think, "Eh? If I throw it away, I won't have anything left." So we keep on carrying it. We aren't willing to throw it away.</p>
  
  <p>Even if someone tells us, "Come on. Throw it away. It'll be good like this, and you'll benefit like that," we're still not willing to throw it away because we're afraid we won't have anything left. So we keep on carrying it until we're so thoroughly weak and tired that we can't carry it anymore. That's when we let it go.</p>
  
  <p>Only when we let it go do we understand letting go. We feel at ease. And we can sense within ourselves how heavy it felt to carry the rock. But while we were carrying it, we didn't know at all how useful letting go could be.</p>
</blockquote>

<p>One might fear to try it, but than it's known, possible to make it a valide direct perception again.</p>

<p><em>[Note: This is a gift of Dhamma, not meant for commercial purpose or other low wordily gains by means of trade and exchange, directed to Nibbana, to simply let go]</em></p>
