These grip strength trainers are silicone rings that you can squeeze while sitting at your desk, watching TV, or doing any other task during which your hands are free. As shown in the product pictures, the black ring is the stiffest, followed by the gray ring, and then lastly followed by the white ring. I recommend starting with the white ring and working your way up to the black one gradually so that you don't strain or overly fatigue your forearm and finger muscles.
I have been using these to help with an ongoing pain from golfer's elbow (which I think I developed in the gym from gripping heavier dumbbells). So far, they have been helping and have reduced the pain along the inner side of my elbow significantly. I have also noticed results in the gym, specifically with my grip strength during deadlifts or other pull exercises. These are probably also useful for rock climbers or others who require high grip strength.