I am not quite sure what you mean exactly, but i will try to help. When saying "that's it", it can be used to state your fed up with something, for example.
That's it, I've had enough of your complaining.
it can also be used to mean you are through with a task or almost through.
That's it, I am finally done with mowing for the day.
or, I only have one more thing left to do, and that's it.
You can switch "that's it", for "thats that" in most cases they are usually interchangable.
For the seond question, explain to me is the correct form.
and if that doesnt help, give me some more information and i'll try to help you more.