What is the meaning of the expression "that's the top and bottom of it"? In which situations can it be used?
The phrase simply means "that's everything, the whole story, there's nothing more to be said.
As Robusto pointed out,
that's the long and short of it means the same thing, and is much more common and well-understood in American English. I don't know how common the
top and bottom of it is in British English.
It means that is the extent of the subject, covering all aspects. Another phrase meaning the same thing is:
That's the long and the short of it.
Use it in situations where you feel you have explained a thing in its entirety (or nearly) and mean to convey that you have said enough on the subject for your listener or reader to understand at least the essential points.
The expression means the summary of something or the final conclusion. According to the Longman dictionary:
"the top and bottom of it": the general result or meaning of a situation, expressed in a few words:
He's trying to embarrass you, that's the top and bottom of it.