This achievement is for escaping 100 cop chases.
You may be able to get a good chunk of this done through natural gameplay depending on how much you like racing at night and getting into pursuits. However, there's a very easy way to evade the cops if you're ever in a jam.
In the below video, I outlined a method how to get quick and easy REP:
You'll notice that about halfway through, the cops start chasing me. If you go to the activities located on the map and follow the path I take, you'll see a jump located past the jump over the yacht. When you jump over this, any cop chasing you will wreck and you'll start escaping the cops. Just keep repeating this until you get your 100 escapes.