_root refers to the main timeline - the very top level of your movie. _parent refers to the movieclip the current one is embedded in.
So for example say you have 2 movie clips - movieclip1 is on the main stage, movieclip2 is inside movieclip1. Here _root will be the main stage, and if you make reference to _parent inside movieclip2, it will be actually referring to movieclip1.
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