A MAC address is a (supposedly) globally unique hardware address of an Ethernet network card. It's a hex number, for example 00-0D-56-68-D2-D8, programmed by the manufacturer when the card is made, and it will be different on every card. It's used deep in the depths of networking for directing network packets to their destination.
If you type ipconfig /all on a Windows PC will tell you what yours is (Windows calls it the Physical Address).
edit: the MAC-address Access Control List (ACL) that i'm talking about is intended to say that only certain specified cards may access the network. If MAC addresses really were unique then this would be foolproof, but, unfortunately, many cards allow the MAC address to be misreported, so once you know an allowed MAC address you can change yours to match