What is MAC address?

I purchased a modem/router that is nearly identical th the one I am renting, but altho the WIFI cable  and internet worked, the phone would not.  Finally had a technician who came and reinstalled the old rented box and got the phone to work again.  But I received a tip from someone in the industry, who said this often haopens when Xfinity does not input and activate the new modem's MAC numbers. If this is true I won't have to return the purchased box and get a new one to install in two weeks.  Does anyone have experience with this?