A bump may refer to any of the following:
1. When referring to a message or forum post, bump or /bump refers to an action someone does to move a message or post back to the top of the list. Most message boards, forums, and similar services only display the last 10 to 30 posts made. If a forum is popular, your post may be buried to where most users are not going to read or see it. By typing something new in a message such as "bump," that message will be moved back to the first page allowing other people or the forum administrator to see it.
It's a good forum etiquette to not bump a post unless it's been over a day without a reply or is several pages deep.
2. A bump is a description of a physical difference on a flat surface. For example, on all computer keyboards, there is a bump on the 'f' and 'j' keys to help position your hands correctly when typing on the keyboard.