thats a film burn, thats an effect that comes from a 16mm, or some sort of film camera. Then people just overlay it on a clip if its a digital shot usually. there may be a way to get that effect in FCP though, havent really played w/ it.
go up to the effects tab then go to video filters then go to image control the go to ganna correction....thats how i do mine like jeff said there are many ways to do it