MAC address is a unique identifier assigned to NIC. It is used for IEEE 802 networking technologies: Wi-Fi, Ethernet, and Bluetooth. Within OSI it's used at the Data Link layer.

MAC addresses can technically be changed, just like IP addresses. For example, with ifconfig on Linux.

MAC structure:

Each MAC address consists of 48 bits or 6 octets.

In the first octet the last 2 bits are significant: one determines if the address is universal vs local. Universal address is assigned by the manufacturer, while the local one is set by the network administrator. The other bit determines if it's unicast vs multicast.

Special addresses:

FF:FF:FF:FF:FF:FF - broadcasting

33:33:00:00:00:00 - limited broadcasting