ERP stands for exposure response prevention therapy and is a form of CBT. The idea is that you expose yourself to things you that cause anxiety, starting off slow, and work towards not responding with compulsions. It is to retrain your brain to stop seeing the thoughts/situations as threats is my understanding. However, I am no expert, that's just my understanding. Here's a link from the international OCD foundation (and there's some other info there you might find useful) International OCD Foundation | How is OCD Treated?.