The Amazing Stickie likes to protect her knees. She knows that the best way to do that is to strengthen the muscles around her knees. Band walking helps create knee stability from side to side.
There are lots of kinds of exercise bands. In the picture, Stickie is using the kind that looks like a tube with handles on the end. She stands with her feet on the tube and holds the handles to put tension on the band. It is also possible to do this exercise with the kinds that look like giant rubber bands. In that case, Stickie would put both ankles inside the rubber band (carefully, so as not to lose her balance!).
Stickie lines her toes up with a line on the floor. She works out in my gym, so she picks one of the lines between the floor tiles and uses that. This is not necessary, but it is instructive because many people doing this exercise find that they creep backwards at an angle as they proceed.
Of course, Stickie is standing with amazing posture. She steps one foot to the side and then brings the other foot to meet it. Simple enough. The thing is, she does this with control, not allowing the band to snap her second foot over to her first foot. She walks about ten steps in one direction and then returns in the other direction, working both sides. For those who really want to know what muscles are working, the step out works the abductors and the step together works the adductors.
Three sets is usually sufficient.