Symbols are elements you reuse within a movie to reduce file size.
It is a way to group the image and keep it as an object.
Symbols are used for Motion Tweening.
Types of symbols include graphics, buttons, movie clips, sound files, and text.
TO CREATE A SYMBOL
To convert a drawing or an image into a symbol, right-click or use a Selection tool to drag an area around the image and select "Convert to a Symbol" .
Give it a name.
Select a Type: Either Movie Clip, Button, or Graphic
It will now be accessible in your library.
EDIT A SYMBOL - To edit a symbol, double-click it.
A library is where you store and organize symbols.
When you drag a symbol from a library to the Stage, you create an instance of the symbol.
Easy editing: If you change the symbol in the library, all instances of the symbol are updated automatically.
Smaller file sizes: Symbols are downloaded only once, regardless of the number of instances you've included in the movie. This reduces the size of your finished movies.
You access your library by clicking on the library icon to open the library panel or by going Window > Library.