“BSA” bottom bracket shells and cups are named after the Birmingham Small Arms Company which created the thread standard used in this system.  It’s also referred to as “British” or “English” style and is the most common type of system found on modern bicycles.  The “external” designation refers to the fact that the bearings will sit outboard of your frame’s bottom bracket shell as opposed to being tucked inside the shell as you would find on some styles of BSA bottom brackets.