MAC addresses are 6 octets, i.e. 48bits. They're displayed as 6 fields of 2 hex digits. These addresses are used by hardware devices to know where to send frames at the lowest network layer (link layer, AKA layer 2). Since MAC addrs are only used by hardware, the original host's MAC is not sent along the entire life of a datagram. Each hardware device that performs routing has it's own MAC, so firewalls, routers, intelligent bridges, etc all have MACs.
Is it the address of my ISP gateway?
Close, it's the MAC of the internal interface of your broadband router.