I have just applied for an F visa and plan to be in Mainland China for about 2.5 months. Hong kong?

I will be arriving in Beijing and then be around Anhui. I am interested in visiting Guangzhou and Hong Kong. I'm curious if I would be able to visit Hong Kong and then renter mainland? I believe my F type is multi-entry (waiting to pick it up). Also, what does multi-entry really mean? Can I enter mainland multiple times during the 90 days, or is there a waiting time before I can renter? Thank you!