Q:

What does the “‑to” ending mean in most English words that end in “‑to”?

Today I saw this word — hitherto — then I searched in the dictionary to
find that it means "up until that time". It looks a little weird since
my first seen, and I rarely see words ending with ‑to except to,
auto and tomato which look more normal.
I found all words ending with to from here, but this site only
says words ending with to are good for playing "Words with Friends"
word game.  Maybe because my native language is not English, I know
almost nothing about all the words listed on that site, so I wonder if
the ending ‑to has some common special meaning? If yes, it can help me
to remember the words that end with to.

A:

I’m not sure what you mean by asking what the meaning of words that end in
-to is, because -to is not really an English suffix in the general
case.  As you note, it occurs in many words with no overlap in sense, like
alto, auto, biscotto, burrito, canto, cornetto, ghetto, grotto, potato,
lotto, magneto, mosquito, panto, photo, presto, recto, veto.  Many of those
derive from Latin or its children, but their -to has no meaning of its own.
Nonetheless, for your case of the adverb hitherto, there actually is
something here behind it.  Just as with adverbs like today, tonight, and
tomorrow, the to part was originally part of a longer adverbial phrase
involving a preposition that was originally worn down into a single word in
examples like hereto, hereunto, hitherto, hitherunto, thereinto, 
thereonto, thereto, thereunto, thitherto, whereinto, whereto, whereunto,
and whitherto.  
In all those, to represents what was once the preposition to.

A:

A good test is to see whether you can break off the "to" and still have a valid word.  In the case of "tomato" and similar words, you can't:  "toma" isn't a word, so the "to" you broke off isn't either.
But "hither" is a word ("in this direction"), so the "to" is also.  In combination they don't necessarily have a meaning that can be predicted from the meanings of their parts, but there's usually at least a distant relationship.

