|mac in a router's wifi section is referring the actual "address" of each device connecting to a/your router. mac = media access control.|
Each wifi device has a unique one, likewise any ethernet adapter too.
In a router's wifi security section there is an option to enable "mac filtering". It allows two options when enabled: allow "only" the mac addresses listed to access the wifi; disallow those listed. Other option is mac filtering is "off".
It allows a tighter control over who can access/use the wifi. A different SSID to the default is useful, as is an access key (often incorrectly called a password) that is different to the default one, will also help restrict/control access.
If using mac control option you need to know the individual mac address for each device you will allow to access/connect.