'That's it!' is an expression used to show approval, encouragement or to indicate that something is correct. You can use 'that's it' to show that something is completed or finished. "That's it for me, I'm afraid. I can't eat any more food!", "That's it, I've finally finished my essay." It can be used to say that something is correct, or that something is sufficient. "Could you close the curtain? The sun is in my eyes. That's it! Thank you." "I just came to the park for a walk and that's it." "Do you spell it C-I-A-R-A?, Yes, that's it!" 'That's it' can also be used in frustration to say you are angry or have had enough of something. "That's it, I've had enough of this job. I quit." In the same way, you can say 'that's enough' to indicate that you wish for something to stop. "That's enough. No more messing in the house. You will break something."