It is like I will remember this

Monica is referring to a list of all the people with whom she is angry with, or against she holds a grudge for having upset or betrayed her in some way. When most people say this, they do not literally mean that they keep a written list. Rather, it’s a funny way to say to tell someone you are upset with them and plan on holding a grudge (not forgetting about it). However, since Monica has a very obsessive personality, we as viewers wouldn’t put it past her to literally keep such a list, which adds to the humour of her comment.