NOT A TEACHER
I've never heard of 'an ace in the hole' either, I think it must be US English. I do know 'an ace up my sleeve', though, in British English. This could go in the following sentence:
When negotiating with her manager for a pay rise, Mary had an ace up her sleeve.
In my example, Mary had something (that her manager was not aware she had) that would help her get a better pay rise. Perhaps she had just made a really big sale for her company?
It might have a darker meaning: perhaps Mary knew that her manager was having an affair, and was threatening to tell his wife unless he gave her a pay rise?
Basically, it means having some kind of 'ammunition' that will help you achieve something, that your opponent is not aware you have (as if you had the ace card in a game of poker).