Corner Bead

The metal or vinyl pieces that are applied to outside corners to make a nice straight line and a finish that is not going to easily chip off. Source: Jason Lebeau a 24 year drywall industry veteran.

A corner bead is a protective trim, usually metal or vinyl, installed on the outside edges of drywall corners to reinforce them, prevent damage (dents, cracks), and create a perfectly straight, smooth, and seamless 90-degree angle for finishing with joint compound, ensuring a professional look. They provide structural support and cover imperfections where two drywall sheets meet.