Defination -- MAC -> Media Access Control -> You must login or register to view this content.
Every company who sells products that can connect to the internet and/or a network has a NIC device. That is a network interface card. This applies to both a ethernet (wired) and wireless too.
The MAC address is hard coded into a ROM ship on that device at the manufacturer. It can not be changed nn a hardware level but it can be changed at a software level. Computers communicate via MAC as well as TCP/IP or UDP. If a PC don't have an IP, it has to identify the other PC by its MAC.
To keep things organized, those companies register/reserve different ranges for mac addresses in the same way your ISP reserves an IP range for you to buy home internet. There is a portion of the MAC that they reserve called the OUI.
The first 3 sections of the MAC is the OUI. So if my mac is AA:BB:CCEE:FF then AA:BB:CC would be the OUI of the manufacturer.
Knowing this I can go to an online mac generator supporting prefixes, put in the OUI, and get a new h4x0red MAC to mod with ..