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.
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.