What does that's it mean?

The phrase "that's it" is often used to indicate that something is complete, finished, or conclusive. It can express agreement, satisfaction, or finality regarding a situation or decision. For example, after explaining a point, someone might say "that's it" to signify they've said all that needs to be said. It can also convey a sense of resignation or acceptance when a person has reached a conclusion or made a choice.