It looks like an easy exercise, but tricep dips is something that few people can do in their first 2 or 3 months of training.
You basically rely on your triceps to lift your whole body weight. This is why some gyms provide a pad and a stack of weights to help you with the lift.
Basically you stand with your knees on the pad (then this depends on which type of dip stand it is) and choose a weight on the stack.
You will then have to lift your body weight minus the selected weight. The pad will push up helping you to perform the exercise.
Tricep dips involves also two important muscle groups like lower chest and shoulders, and that makes it an effective compound exercise, which is sometimes recommended also as a lower chest exercise too (if you use a wider grip).
Tricep Dips Details
PRIMARY MUSCLE: Triceps
SECONDARY MUSCLES: Lower Chest, Shoulders
EQUIPMENT: Dip Stand (Optional: weight belt + weight)
Depending on the width of your grip and whether you bend forward or not, tricep dips can become a lower chest exercise.
INITIAL POSITION: Grasp the bars about shoulder width and fully extend your arms upright. Some dip stands have 2 small feet platforms so that you can reach the initial position and rest when you finish. You can cross your legs (I find it gives more stability).
MOVEMENT: Slowly lower your body and stop when you upper arms are parallel to the ground (90 degrees with your forearms). Then push up focusing on your triceps only until you reach the initial position again.
BREATHING: As always, exhale when you push up and inhale when you lower your body.
TIPS and ERRORS: Try to keep your body straight without bending forward and don't go wide with your elbows. A wider grip focuses more on your lower chest than triceps, even more if you bend forward.
Oh, You're a Strong Guy???
In this case if you can do 10 reps or more using just your body weight you may want to consider adding extra weight to increase muscle mass and maximize the results.
Every gym should be equipped with a weight belt. Then you can put the belt on and attach more weight to it.
At the beginning it will be difficult to keep the balance because of the weight swinging, but if you get to this point you will get big arms quickly because this is a very effective tricep compound exercise.