It's an expression that is used to conclude something and to say that there will be no more after this. It could be at the end of a speech, a class, a performance, a meal, even a relationship. Another meaning, when used as a strong exclamation, could be similar to "Eureka!", when a missing clue finally appeared which explains all the questions that have been asked before.

Please provide some context. It could mean a variety of things.

It could mean many things. The first that comes to mind is that if this answer doesn't explain enough. You could respond, "so that's it? That's all he's got to say about that?" It can be used to express disappointment in something that you have purchased ("So that's it? That's all it does?"). It could be used to confirm that something is finished or something is done. For example you could inquire if a job or sale is finished ("So that's it? Do you need anything more today?"). You could inquire if a kid is done with school work ("So Johnny. That's it? Any more homework for tonight?")