A MAC (Media Access Control) address is a unique identifier burned into a device's network hardware — every Wi-Fi and Ethernet interface has one. Unlike an IP address, which changes as a device moves between networks, a MAC address is tied to the physical hardware itself, and its first half identifies the manufacturer. On a local network it is how devices are told apart.
For OSINT, the MAC address of a wireless access point (its BSSID) is especially useful: public wardriving databases map millions of these identifiers to physical locations, so a known BSSID can be geolocated to an address. Because true MAC addresses are only visible on the local network, modern devices increasingly randomise them to resist exactly this kind of tracking.