user64788: I understand that it starts bc1.
But 'q' is not what I understand. Can someone explain to me?

user38482: There is of corse BIP173, which explains bc1 for better readability. 
More interesting is a thread in bitcointalk, where Greg Maxwell explains details. 
Searching the forum here and bitcointalk for bech32 reveals lots of interesting facts :-)

user48884: Immediately following the human readable part and separator of a bech32 address is the witness version number. For current bech32 addresses, that version number is 0. According to the encoding table here, the value for a 0 byte is q. Thus addresses will begin with bc1q.

user51948: q is the Bech32 encoding of the number 0. For a segwit address, as defined in BIP 173, the number immediately following the separator is the witness version. In this case, bc1q, the q means it is a witness v0 address, which is the only type currently used but allows for easy future addition of new versions

64788: <p>I understand that it starts bc1.</p>

<p>But 'q' is not what I understand. Can someone explain to me?</p>
 38482: <p>There is of corse BIP173, which explains bc1 for better readability. </p>

<p>More interesting is a <a href="https://bitcointalk.org/index.php?topic=2624630.msg26762627#msg26762627" rel="nofollow noreferrer">thread in bitcointalk</a>, where Greg Maxwell explains details. 
Searching the forum here and bitcointalk for bech32 reveals lots of interesting facts :-)</p>
 48884: <p>Immediately following the human readable part and separator of a bech32 address is the witness version number. For current bech32 addresses, that version number is 0. According to the encoding table <a href="https://github.com/bitcoin/bips/blob/master/bip-0173.mediawiki#bech32" rel="nofollow noreferrer">here</a>, the value for a 0 byte is <code>q</code>. Thus addresses will begin with <code>bc1q</code>.</p>
 51948: <p><code>q</code> is the Bech32 encoding of the number 0. For a segwit address, as defined in BIP 173, the number immediately following the separator is the witness version. In this case, bc1q, the q means it is a witness v0 address, which is the only type currently used but allows for easy future addition of new versions</p>
