Mental event is a catch-all phrase that means
a particular occurrence of something going on in the
mind. So say Mary is walking through a park and she
sees City Hall. That instance of seeing City Hall is an instance of
perception -- something that is supposed to be going on in Mary's mind. That instance of seeing is a mental event. It’s an
event because it is something that happens; and it’s
mental because it happens in someone's mind. If Mary thinks to herself, "I am a human being," that thought is a mental event; if she feels happy after doing well on an exam, that’s a mental event. And so on.
See also: quale