A compression shirt is a tight-fitting, very breathable shirt that is usually worn under a regular Rugby jersey.
Compression shirts have several roles:
They are made of a durable, elastic-like material that helps wick moisture away from the body, keeping players cooler and dryer.
They cut down on excess muscle vibration which can cause extra fatigue
They prevent chafing.
Some compression shirts are also made for cold weather and keep the body extra warm during play, helping prevent injury to the muscles and joints.