You can also use _parent to target the level exactly one above the current level. So, if you are one movie clip down from the root level, and you use _parent, it is the same as using _root. However, if you are two levels down, _parent means the level above, whereas _root means two levels above.
It can help to number the levels. The root level, which is the main timeline, is level 0. A movie clip on the root level is at level 1. If there is a movie clip inside that movie clip, it is at level 2. From level 2, _parent refers to level 1, and _root refers to level 0.
So what about the other way? If you are at level 0, and you want to refer to a movie clip named “gears”, you have already seen that you can refer to it by name. You can also use the term _root followed by square brackets, with the name of the movie clip inside it. Here are two lines of code that mean exactly the same thing, provided they are at the root level:
gears.gotoAndStop(7);
_root["gears"].gotoAndStop(7);
Taken From: Sams Teach Yourself Flash™ MX ActionScript in 24 Hours
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