Use this phrase when you're finished discussing all of the topics that you wanted to discuss in some kind of meeting. It means "that was all of the topics that I can remember that I wanted to discuss". Add "I think" if you're not completely sure whether you've discussed everything.

For example:

A: OK. Anyone have anything else they wanted to add?

B: Nothing for me.

C: No, I think that's about it.

You can use "That's about it" when you're listing things and you think you're at the end of the list. For example:

Can you pick up some cheese, avocados, carrots... maybe a case of beer... and that's about it.

You can also say "That's it" without "about":

OK, I think that's it.