if you prefix anything by _root. whatever follows applies to the main timeline (an important thing to remember is if you use _root in a movie loaded into a movie clip, _root. will target the main timeline of the movie the file is loaded into and no longer it's own main timeline) so for example,
sends the main timeline to play the frame "framelabel"
finds the bytes loaded in the main timeline.
_parent can be used in the same way as _root except instead of going back to the main timeline it targets the timeline containing the movie clip with the action in. (so if you have a movie clip placed on the main timeline within that clip, _root and _parent will both target the main timeline)
you can string together sequences of _parent's to navigate back through multiple nested clips.
this means the current timeline,
would send the current timeline to play frame 2.
_global is an object, it can be used to hold variables so they are available to any timeline within the movie. for example,