Used to indicate agreement with or confirmation to the previous speaker's remark.

(US, African-American Vernacular) Indicates complete agreement.

(idiomatic, colloquial) Used to introduce the main point or issue, especially as a refutation or negation to a statement made by another speaker.

that's it; that is the end; there is no more

(slang, obsolete) That's it; that's the spirit; expressing approval or encouragement.

Alternative form of it's all Greek to me