Procedures and Functions consist of a number of script commands. Procedures can be run using the "Run a Procedure" script command from the "Run script" category. Functions are like procedures, except that they return a value. You call a function by surrounding its name with "$" characters in any parameter - when this parameter is read, the function is called, and its return value is placed in the parameter.
By using procedures and functions, you can create sets of commonly-used script commands that can be accessed from any point in your game.