What is a 'symbol' in Adobe Flash?

A "symbol" is anything added to the Flash library that may have been created or imported to the Flash stage. The purpose for creating a stage object as a symbol is so that it can be used as many times you require without using extra resources, as well as reuse it in a variety of ways where the symbol can change within itself based on how ActonScript defines its parameters.

Let's say you wanted to create an animation with about 6 characters and a suburb with a few houses. Create the graphics and make key items as symbols to be reused again and again, such as a roof, a character's head shape, hair, trees, cars, clouds, etc. Any of those items can be dragged onto the stage from the symbol library and changed on the stage without disrupting the parameters of the library symbol.

You could make different sets of facial features, turn them all into symbols and animate the results in a multitude of combinations on the stage without the need to redraw them all. The end result is a lightweight Flash file that has fewer library resources than the stage would implicate based on the complexity of symbol usage.