This idiom or expression can best be explained through examples:

Accepting the loss of something, as in:
When someone says: If you left your wallet at the casino you can kiss it good-bye, they mean something like:
Forget about it because you won’t see it anymore!

Not doing something anymore, as in:
When someone says: I’ve kissed college good-bye, they mean something like:
I won’t be going to college anymore.