These are a valuable exercise for targeting and strengthening the triceps, the muscles at the back of your upper arms. It is typically performed using a cable machine found in most gyms. It's an effective exercise to strengthen and tone the triceps while offering the benefit of constant tension on the muscle throughout the entire range of motion.
First, place the cable pulley at chest height, then you should position yourself perpendicular from the pulley and grab the handle in the hand furthest from the pulley. Then have your elbow up to the side at shoulder height and your hand pointing towards the pulleys so the cable is crossed.
This is the starting position.
From here extend your hand out to the side, by extending at the elbow until the arm is fully extended or as far as you can. After this, slowly return the arm back to the starting position. Repeat on the other arm. Once you have done both this is one repetition, now repeat this for the desired amount.
If you have limited gripping ability try using the Active Hands Gripping aids or D-Ring Aids clipped straight into the carabiner to make this exercise as safe as possible.