Hmm. I thought it was for SilverCreator. GameMaker will give you a random number from 1 to the number you put in the command.
x = RANDOM 3
will give a result from 1 to 3
If you put multiple parameters after RANDOM then it will give you a random number of any of the numbers listed.
x = RANDOM 5 10 15 20
will give a result as either 5, 10, 15, or 20.