zrandom is a set of scripts to make random in game maker independent for each instance. Its a good set of scripts for procedural content and stuff like that.
It works basically just like the game maker functions. with the difference that one random call in one instance doesn't affect other random calls in another instances. You'll only need to call zrandom_init() before calling any of the zrandom scripts. The idea is that the names of the funcions are similar to the game maker ones, with "z" as a prefix. For example: random(10) = zrandom(10) random_range(10,30) = zrandom_range(10,30) etc.
Keep in mind that this is not a perfect random generator. Actually is a pretty simple set of scripts and it uses code that you can easily find on the internet. Just search for Linear congruential generator.