A MAC address, also know as a physical address or a
hardware address, is a unique numerical value assigned to a network
interface card (NIC) during the manufacturing process or by setting
jumpers or switches during network installation. One part of this
address is assigned to the manufacturer by the IEEE (Institute of
Electronics Engineers) and is common to all components from that
manufacturer; the second part of the hardware address is a unique
value assigned by the hardware manufacturer.
For example, the physical address (or MAC address) of ethernet adapter can be 52-54-AB-20-88-AD. TrafMeter allows to create rules with MAC address as source or/and destination addresses.