Stop This action will stop your movie from playing. This doesn't mean that you can't go back and make it play again. It will just stop until you give it an action to play again. Movies by default loop constantly. So to stop it, you simple insert a 'stop' action on the last frame.
High quality is slower on older computers but look a ton better.
Low quality will process faster on older computers and even on new ones, but the lines will look jagged and nasty. So where the movie gets a little sticky and slow it will switch to low quality to compensate for the lag. This action will stop all the sounds in the movie that are currently playing at that time. Typically that would be a background sound. It won't disable them permanently though.