_parent is the location that the current timeline is in, take this for example:
A flash timeline with a movie clip called ‘A’ inside it and inside that clip there is another movie and its called ‘B’, and say I put this code in frame one of clip ‘B’ to control clip ‘A’:
That makes the gotoAndPlay built-in function refer to the timeline that contains the clip that you placed the code in! Its like a ladder, lets say you start at the top and you are down 12 steps, and want to get to the step above it, would you use _root to start at the top and work your way down through 11 steps or use _parent and automaticlly make the code effect the step above you. If you don’t get why this is important then you have never have a clip embeded in a clip embeded in a clip…(and so on for 20 timelines) and your on the 20th timeline and want to refer to timeline number 19 then you ate literally screwed if you want to control all those embeded clips from the code in the 20th timeline cause you would have to use _root and work your way down but with _parent you can use it as many times in a row to work your way to the timeline you want:
If your on Timeline 20 and want to get to timeline 18 then you would use _parent twice. My fingers are tired so I will stop typing hoping you get the idea and if not then maybe some should write a tutorial on different built in commands and functions!
Hope I helped, but I don’t very often so hope you got lucky and understood!