Say you want to sell something so you create an ad and post it. As soon as you post it goes to the top of the list of ads in that category. As time goes on, if nobody responds to the ad it will eventually drop down the list (as new ads get posted above it) and basically get 'stale'. If someone responds to your ad, that response will make your ad jump up to the top of the list, or get 'bumped' to the top, which is fine and dandy. The bumps that they are referring to are the bumps done by the seller to force their ad to the top of the list. The seller simply writes 'bump' (or any number of other things) and bam, that ad goes to the top of the list. It can be annoying if people do it too often so the rules you read are an attempt to limit the amount of bumps a seller does for their ad.
Hope that helped a little.