SWOLF is a swimming metric that provides an insight into swimming efficiency. It combines the number of strokes per lap and your time for the lap, and is the sum total of the two. For this reason, SWOLF scores are different for every stroke, and can also vary based on the lap length. SWOLF is a personalized score and lowering it over time (for the specific stroke type and lap length), can be an indication of improved efficiency.
Eg. If you swim 25 yards in 30 seconds and take 15 strokes, your SWOLF score will be 45 (30 + 15).