Well to bump does a few things...
... first "bump" means to bump a thread to the top of the stack in the "Latest Posts"....
it a way keep a good story in play when comments might be slowing down...
even if you have a thread that been dormant for months, if you bump it its gets seen again in"Latest Posts"
but it also a quick way to mark a thread for a later read because your bump will come up under a “My Comments” search
You see that when someone posting “bump for later read”
but also you can bump a thread as a recommendation as in “good read bump”
Also sometimes a bump is really a "ping"
(and a ping here is if your not sure is a post to several people you want to give a quick heads up to about a thread because it will come up under there “My Comments” search...
the term "ping" is a take off on IP ping command used on a network to "ping" a devise to see if its alive and there)