Many actions make reference to a 'Target'. The target is generally the Movie, Sprite or Object that the Action applies to.
The target can often be selected via a pull-down list in the Script Panel. Alternatively it can be typed in long hand.
The Object Synonyms '_root', '_parent' and 'this' can be used when the target refers to a Scripting Object. Note that slash notation is also accepted and is converted to the Object synonyms.