It's an IPv6 address that incorporates the MAC address of an interface. Here's an example.
If the MAC address is: 00:04:de:a8:cc:1b
We first stick FFFE in the middle of the MAC address. So we get: 00:04:de fffe a8:cc:1b
Then we invert the 7th most significant bit. So we get: 02:04:de fffe a8:cc:1b
Then the link local prefix is prepended and so we end up with: fe80::0204:deff:fea8:cc1b