A Media Access Control address (MAC address) is a unique identifier assigned to network interfaces for communications on the physical network segment.
A host cannot determine from the MAC address of another host whether that host is on the same link (network segment) as the sending host, or on a network segment bridged to that network segment.
On broadcast networks, such as Ethernet, the MAC address uniquely identifies each node on that segment and allows frames to be marked for specific hosts.
It thus forms the basis of most of the Link layer (OSI Layer 2) networking upon which upper layer protocols rely to produce complex, functioning networks.
MAC spoofing
MAC addresses are used for routing packets between physical devices (i.e. network interface cards) on networks (like Ethernet networks).
