They are a very time saving feature of flash. You can edit the symbol in the library and thus editing the same symbol wherever it is located in your movie. It is very easy to change the position, color effects, and size of the symbol.
Symbols can either be buttons, graphics or movie clips.
Symbols introduce a numerous advantages to animation. First, they are not time consuming, they make it easy to edit repeating elements in a movie. Using symbols throughout the movie reduces the file size significantly Furthermore, the symbols are loaded only once so the playback time of the movie is reduced. Using symbols as often as possible in your experience will improve your performance. This significant change will further make your experience with flash worthwhile and enjoyable.