Every Ethernet network card has a unique MAC address. The Ethernet protocol uses MAC addresses when sending information directly from one device to another device.
An example of this would be when one workstation sends traffic to another local workstation. Or when a workstation sends traffic to its default gateway.
This network traffic is broken into chunks of information called frames. The frame includes the sender’s MAC address and the MAC address of the destination device.
If the network traffic needs to pass through a router, the router will change the source and destination MAC, to suit the next hop in the network. This is called frame rewrite.
In hardware, each network card has its own MAC address. This is also called the Burned In Address, or BIA, as it can’t be changed.
However, the operating system will sometimes ‘fake’ a different MAC address. This is becoming more common in smartphones. They do this when connecting to public WiFi, to make themselves harder to track.