In c++ terminology (can also be in c) , a handler (also called an exception handler) is a piece of code that manages program events : it catches the event and reacts to it.
Similarly (as being said) you can also have signal handlers. You tell your program that whenever a signal appears, it has to be treated by sig_handler. Then all the error/signal/events/.. are handled in the same function.
The same applies for an interrupt, as yous said. When an interrupt is raised, the handler deals with it, does the action and returns.
A handler can also be an overclass. Rather than using directly a door, you will use the handle to open the door. In that way a handler can also be a helper and this helper can be reused for other objects ( a trunk, a window,..) , it centralizes the entry for an action.