This is a 4awg tinned copper ring terminal. These work great on my #FAM branded cable.

Bare copper is fairly resistant to corrosion, but when used in humid or wet environments, it can quickly become damaged.
By coating bare copper with a metal alloy, like tin,the copper's natural properties are strengthened giving it a longer life.
These are available in 5/16" and 3/8" rings. The 5/16" fits my grounding bolts perfectly, and 3/8" usually work well for hooking wires up to the battery.
Sold individually.