An eye bolt is a bolt with a loop at one end. They are used to firmly attach a securing eye to a structure, so that ropes or cables may then be tied to it. A 5/16" x 5" steel zinc-plated eye bolt refers to an eye bolt with a thread diameter of 5/16 of an inch and a length of 5 inches. The zinc plating provides a layer of corrosion resistance to the steel, making it suitable for outdoor use.