In more detail
It is written as six pairs of hex digits, such as 00:1A:2B:3C:4D:5E. The first three pairs, the OUI, identify the manufacturer. Two bits of the first byte have special meanings: the lowest bit marks a multicast address (0 means unicast), and the next bit marks a locally administered address set by software rather than assigned by the maker. Unlike an IP address, a MAC address does not change with the network, although phones and computers now often randomise it for privacy. It only matters within one local network: routers replace it at every hop.
Why it matters
Wi-Fi MAC filters and DHCP reservations are keyed on the address, so randomisation can make a known device look new. MAC filtering is weak security because any address can be copied and spoofed.
Example
In 3A:1F:00:12:34:56 the first byte is 0x3A = 00111010: the lowest bit is 0 (unicast) and the next is 1 (locally administered), so it is a random or software-set address and no manufacturer lookup will match it. The address 00:1A:2B:3C:4D:5E also appears as 00-1A-2B-3C-4D-5E (Windows) or 001a.2b3c.4d5e (Cisco). Find the maker of a real address with the MAC Address Vendor Lookup.