Open Question: I don't understand how PPO insurance works?

I've been looking for a new dr., as I have new insurance. I found a dr. who I would like to see but she is not listed as a ppo provider for my insurance. The dr's office is telling me that that's ok, as one of their other dr's is a covering provider and they can just bill under her, although I would be seeing the other dr. Does that make sense? I just get nervous I'll get bills with things not