As others have said, it’s a way to bring a thread back up to the top of the list. Done in an effort to get others in to provide answers if the “bump-er” can’t provide them personally. Or if you just want more people to see the thread (I.e. if it’s a for sale thread)
It puts a post back at the top of the list where more people see it on page one.
Sometimes it also takes an unanswered question off the unanswered question list and disappears it to be lost in the many posts made every day. The only answer I typically get is
Bump