It is an event that occurs when a running program requests to read data that does not exist in the real memory - RAM -, then the program execution will be paused (Interrupted) by the OS, which in turn will look for that data in the "virtual memory", and loads it to the RAM, then the program continue execution normally having the requested data in the RAM as expected.
That is the simplest way I can think of, If you would like to go further to explain the virtual memory, and invalid page fault, things can be a somehow less simple :)