[center]They're the graphics for doodads, such as trees and such. There are also unit sprites. They're basically the animations that SC uses for doodads and units. Only placable with extended editors like StarForge and SCMDraft.[/center]
When you place a a doodad that can be walked behind, that means there is a sprite. it is composed of two things: terrain tiles, and a sprite, determined by it's number. In Staredit, there is no way to change this number, or place one without terrain. However, Starforge and SCMDraft allow you to simply place a sprite at virtually anywhere. This also means that the sprite number can be changed, allowing you to place unit sprites, unit bullets, spell sprites, explosions, and even sprites that were not used in the final game.