Queens and rooks are called major (or heavy) pieces because of their long range and high value — roughly nine and five points respectively. Bishops and knights are the minor (or light) pieces, each worth about three points, and they do the bulk of the maneuvering in the opening and middlegame.
Understanding the difference guides countless decisions: you generally develop minor pieces before major ones, you avoid trading a good minor piece for a passive one, and you remember that two minor pieces are usually worth more than a single rook. Endgame technique also hinges on the distinction, since king-and-major-piece mates are elementary while two knights alone cannot force mate.