Costumes are the different outfits people wear in order to play a role, either for a job (such as an actor on stage) or for fun. Many find that certain costumes, uniforms, and dress-up can be a turn on, by either wearing it and becoming the role they wear or by having it worn by a partner for some fun in the bedroom.
There are many different costumes that you could try in the bedroom to spice things up a bit. Talk with your partner about any that you both want to try to make sure they have it. For example, you might want to see your partner in Princess Leia's Slave Bikini from Star Wars, a sexy sailor outfit, or even a simple ribbon and nothing else, but if they haven't got time to prepare then it might not be available for you! Let them know what you are looking for, or maybe even ask what costumes they have available. There is sure to be something to get you going.
Part of the fun of costumes is the acting. Costumes tie in with role-play very well, as you can be anything you want with a little imagination and confidence to act it out. For some, they feel a little uncomfortable with the acting and simply enjoy wearing or taking off the outfit. If you think you might not be able to do the acting, let your partner know before-hand so that you can both just enjoy the costume itself and what lies beneath it.
If the costume comes with props, why not use them in some way? Let your partner tickle you with a feather duster if they have chosen to dress as a French maid, or ask them to leave on their stockings and hat if they are dressed as a sexy sailor. It is all part of the fun, so experiment and see what different fun things you can do.