Question:What is movie/movie clip symbol? Why is it used? 

Movie clip symbol is an important component in Flash movie. It is a movie within a movie. It can contain many graphic, animation and sound components. We can use them to create entirely self-contained pieces of action that we want to run independently of the rest of the things on the main timeline.

Question:What is registration point? Why is it used? 

The registration point defines the true origin of the symbol. It is "where the symbol is" and the position of the registration point gets exported to the final SWF. We use the registration point to identify the very center of our symbol, because Flash will consider the center of our symbol.