This is a rear trail shield (also called a rear skirt or discharge chute deflector) that mounts to the back of a walk-behind lawn mower deck. It helps contain grass clippings and debris being expelled from the mower deck, directing them downward toward the ground. Replace this part if it is cracked, broken, missing, or no longer deflecting clippings properly, which can result in debris being thrown at the operator or bystanders.