Peers are people who hold titles from this set of descending order of importance - Duke, Marquess, Earl, Viscount, and Baron (and feminine equivalents). These tended to be hereditary, so that a person who the king or queen gave a title like Duke of Marlborough got to pass it down to a descendant.
https://en.m.wikipedia.org/wiki/Peerages_in_the_United_Kingd...