In the sport of archery, an archer will wear a triangular chest protector, what is it for?

You are talking about a chest protector,

The common protections in archery are chest protector, arm protector and finger holster.

The chest protector is also placed on the left side of the chest at the heart to protect the most vulnerable part of the body. In addition to strengthening the support of the arm, the chance of injury to the fingers is also increased. Wearing leather finger cuffs mainly protects the fingers from overloading and reduces the chance of archery injuries. The common protective gears in archery are arm guards, chest guards and finger cots.

Arm protectors are mainly made of plastic and are attached to the lower arm of the left hand with an elasticized wire loop, which on one hand strengthens the support of the left arm and on the other hand reduces the chances of being injured by arrows.

The armguard is made of plastic and is attached to the lower arm of the left hand by an elasticized wire loop, which strengthens the support of the left arm on the one hand and reduces the chance of being injured by arrows on the other.

It is only used for recurve bows and traditional bows!