A pre-keyed explosion is a video clip of an explosion from which the background has been removed for the purpose of inserting the footage into a different background using certain video editing software. It is a type of special effects technique commonly applied during the movie making process.
Chroma-keyed is a term that is synonymous with pre-keyed. Since certain things, such as gunfire or explosives, consistently behave in the same manner, it is not necessary to reproduce their effects repeatedly. By purchasing pre-keyed stock footage, movie makers can save time and money by not having to remove the background of the image themselves.