An externship is a senior year clerkship that you take at a Medical Education Institution other than Saint Louis University School of Medicine and its affiliated hospitals.
Military clerkships are an example of externships.
Many civilians will also want to do one or several externships. A common reason for doing an externship is to audition for a position in the residency program at that same institution. Students applying in the most competitive tier of specialties usually opt to do two externships at institutions to which they are making application. There is some evidence in the Medical Education literature to indicate that those who do two externships have a higher success rate in matching into at least some program in that specialty. What literature there is, would suggest that beyond two externships there is probably no additional benefit to be gained.