Mazin is a Northern lord, the head of House Mazin, and a bannerman of House Stark. He remains loyal to the Starks during their campaign to take back Winterfell from House Bolton, contributing soldiers during the Battle of the Bastards.
Lord Mazin is the head of House Mazin.
In the books
In the A Song of Ice and Fire novels, there is no mention of any House Mazin. The name is an in-joke reference to fellow scriptwriter and friend of the showrunners, Craig Mazin.