( informal )
1.
used to say that sb is right, or is doing sth right :
No, the other one ... that's it.
That's it, carry on!
2.
used to say that sth is finished, or that no more can be done :
That's it, the fire's out now.
That's it for now, but if I get any news I'll let you know.
A week to go, and that's it!
3.
used to say that you will not accept sth any longer :
That's it, I've had enough!
4.
used to talk about the reason for sth :
So that's it—the fuse had gone.
You don't love me any more, is that it?