Pop one string from the stack. If the string is the empty string, then the next actions apply to the main movie. Otherwise it is the name of a Sprite1 and the followings actions apply to that Sprite only. (Note that some actions, like the Set Target action, are always used globally.)
This was the old mechanism used to apply actions to a given Sprite. Since version 5, it is preferable to see Sprites as objects and manipulate them using method calls and variables (see Call Method and Get Member actions.)