A bump may refer to any of the following:
1. With a message or forum post, bump (/bump) is the action of moving a message 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 and prevent most users from seeing the message. By typing something new in a message such as "bump," that message is moved back to the first page, to make sure it's seen.
It's a good forum etiquette not to 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's a bump on the 'f' and 'j' keys to help position your hands correctly on the keyboard.