"That's about it" is an informal phrase you can use to mean, "There's nothing more to say about this topic." For example, if a friend asked what you did over the weekend, you can respond like this:
I did some laundry, watched a few movies, prepared some meals for the week, and slept late on Sunday. That's about it.
When you say "that's about it," you mean, there's nothing more to say about what happened during the weekend.