A movie extra (or "background performer") is a person who fills the background of a scene.  They might be a passerby in New York City, part of a crowd celebrating the end of WWII, a customer in a 1950's diner, or part of a spaceship crew!  Extras fill the background with life, making a scene more realistic.   

For those interested in a film career, working on set as an extra gives you exposure to professional sets as well as supplementing your income.  For others it is simply an interesting experience to be part of the movie-making process.  Whatever your reason for wanting to be an extra, you can be sure that each time you go to set it will be an adventure!