It is an informal expression that can mean that something is finished or complete. Or it can mean that you're accepting a situation that is not ideal.

"Things are as they are" i.e. nothing is going to change, the situation is the way that it is and we all have to deal with it. For instance people use it to show that they're not willing to debate or discuss something further and people just have to accept the situation - "You are going to wash the dishes, and that is that."

It means that things are set the way they are and will not change. Usually it's used when a person won't change their mind. For example: 1. You will go to your room and study. That is that! 2. You can't try any of my ice cream. That is that. Another expression that is very similar to "that is that" is "set in stone" "That is that" usually has a negative connotation but "set in stone" doesn't always have a negative connotation. For example: 1. I got my paper signed and I'm ready to send it in. Everything is set in stone. 2. I'm sorry. We already picked the cast for the play. It's all set in stone. Hoped I helped^^

what situation did you find it?