The meaning of "This is it"

This is it usually means this is what we've been waiting for, or in other words, this is the thing that have been mentioned in the past and was about to happen some time in the future, which is now.

Imagine you are currently having your school leaving exams, you've already written all the papers and you're waiting for the results. Your schoolmate will say:

Well, this is it. Let's see how successful we were.

Therefore, it may be said when a certain thing ends, but it doesn't necessarily expresses an end.

For example, imagine you have hired a painter to paint your house and you've been waiting for 2 months for him to come. On the day of his arrival, moments before he comes, your wife may say:

Well, this is it. Let's see how good this guy is.

It depends on where you use it. It could mean, "This is the limit." as in:

This is it. He's not going to get away this time.

Or it could mean, "this is what we have been waiting for." as in:

Jon, the mail's come!/This is it! I need this!

It is actually quite simple

This is it.

"This", as in this that you perceive and you are aware of, "is it", where "it" is a third person pronoun, refers to some "it" that is already known to the writer and the reader or the talker and listener (something that was discussed or agreed upon earlier or something that is clear from context).

In another words, the key is it and the fact that that it is equal to this that we have, see, experience. It denotes that that it does not extend beyond this - in this way it is associated with the end.

Taking RiMMER's examples - the it is waiting for the results and waiting for the painter; in both cases the statement means the wait is over (it ends) or about to finish.

You can also say "Is this/that it?" if you are not satisfied with how someone finished some task: Is this (that is in front of us) it (the it that we agreed and shook hands on)?

Without any previous context the statement "This is it" is mostly used in movies as famous, but faux, last sentence - referring to a very general "it" - life itself. (It is faux because you rarely see a story where someone actually dies after they said "This is it, we are about to die.")

I think the other answers have done a good job of discussing this is it. Let me comment that a very similar expression,

That's it.

can indeed mean "this is the end." or "that is all there is."

In the idiom this is it, the word this can refer to what is about to happen, while in that's it, the word that can refer to what just happened. So that's it can mean it's over, or paraphrasing, this is the end.

How about This is it in this context, is this implied that we missed him so much and see his performance for a once more after he has passed away so they use this name for this movie ?