Everyone who posted before now trying to be helpful: Only post if you know what you're talking about kindly.
A bump is given its name because it is a post intended to "bump" or move your thread (or any thread for that mater) to the top of the page. So a bump can really consist of anything. Most commonly, they are used (and misused) in the market when trying to gain more buyers/bidders for an item. If in the market, only bump a thread if you follow the bumping rules in the rules section. A thread in OffTopic can be bumped at any time so long as you are adding information or otherwise constructively adding to the thread.
A good general rule for anywhere else on the forum is to not bump a thread that is over two weeks old.
This has been a Toribash forum definition provided by Solax.
See also: Necrobump and grave digging.