A bit part is a small, minor role in a play, movie or production. It usually involves only one or two lines and little screen time. The term comes from the early days of theatre when actors would speak their lines using a device called a 'prompter's box', which contained wooden blocks with letters on them to remind the actor what they were supposed to say next. Each block represented just a few words, so even a small part might only have one or two blocks to remember - hence the term "bit" part.