Actually, bump is an acronym that goes back to the the early BBS (bulletin board system) days. It means: "Bring Up My Post". For systems that support listing topics by the number of responses, or how recently responses are made, a simple response of "bump" (actually, any response at all) will "bump" the topic higher in the listings.
But I view this site via a web browser, and have not noticed any particular impact on message threads related to the number of responses. Maybe it works if you are viewing the site via a news reader instead of a web browser.