Sandra--let me try to explain:
These bridesmaid's dresses have a train on them much like a formal wedding gown. A bustle is something created by a seamstress that sort of 'tucks' up the train onto the back of the dress, so you are not forever tripping over the train. There are usually small thread loops sewn to the back of the dress, and small buttons on the inside seam of the dress, and the buttons go into the loops. Sadly, the bustle oftentimes just serves to make your butt HUGE.
Hopefully that helps, or someone else can explain better.