Each time a thread is replied to, the thread moves to the top of the list in its forum. Posting the word 'bump' counts as a post and serves to bump the thread to the top of the line. As AbbysMom indicated, here on TCS, bumping threads is something that we ask only moderators do. It indicates that the thread contains important info for everyone to read, and we hope you don't miss the message. When all members begin to bump threads just because they want attention or replies, it diminishes the importance of the 'bump'.
People have asked, "If it's so important, then why not sticky it to the top of the forum?" The reason is that the majority of people do not bother to open and read stickied threads.