Set of two Bracoo KS91 "knee fulcrum sleeves" that are meant to guide the knee to one dimensional hinge-like motion with no damaging twisting or flexing in other directions.
I have a meniscus tear and generally deteriorated knees so I got these to hopefully prevent further damage while running and other workouts. I wore them a couple of times for yoga, which was fine, and a couple of times running, which didn't work because they shifted down below the knee.
The size is determined measuring around your thigh 10cm (about 4 in) above the knee. These are size small, for thighs between 37 and 43 cm or 14.5 and 17 inches.