A symbol is a drawing containing geometry (or other entities) which you regularly use (by inserting into other drawings).
Normally symbols contain a small number of simple entities; it is unusual for them to require the use of blocks (or even other symbols) and it is unusual for them to contain more than one layer.
Use File - New - Symbol to create a symbol, File - Open - Browser to open one and Block - Insert Symbol to insert one into the current drawing. The Project Browser can also be used to manage all the symbols (for example organising them into groups).
You can modify the database information for an opened symbol using Database - Symbol.
Some special symbols (such as the grain/flute direction) can be defined using Impact Options - Database Operation.