From the International Paralympic Committee website

Players must wear opaque eyeshades at all times ensuring fair competition. All international athletes must be legally blind, meaning they have less than 10 per cent vision, and are classified as a B3, a B2, or a B1 – totally blind.

Teams are made up of six players, with three members playing at any one time.

The object of the game is to throw a ball past the opponents and into their net to score points. Players stay on their hands and knees to defend their net and score against their opponents.

A goalball court measures 18 metres long and nine metres wide. It has goals at each end covering the entire nine-metre back-line. String is taped to the markings on the court to allow players to feel the lines and orientate themselves.

Ya, I thought the name sounded frivolous when I heard it for the first time