Hooyah!
Dips are prefromed by holding your body suspended vertically with your arms extended straight to your sides. Your hands grip a horizontal tube on either side that is fixed to the floor or on a sturdy stand. Your feet are off the ground, your knees bent at approximately a 45 degree angle back up towards your butt. Then you lower yourself by bending at the elbows, to a point where your arms are bent at a 90 degree angle. Then push your body back up into a locked position. That's one dip.
If you can't do those yet to any significant amount, you can do 'girl-dips', in which you get 2 chairs, put your feet on the edge of one chair out in front of you and your hands just behind you on the edge of the other chair. Your legs are now in a horizontal position and your upper torso is upright, with your hands behind you on the edge of the chair. Now lower your body to the point where your elbows are bent, again at a 90 degree angle, then push back up to an arms-locked position. That's one 'girl-dip'.
I don't have a dip rack at home, I use 2 chairs on a carpetted surface so they don't move around. I also wear a pair of gloves as the chair backs are thin and painful to use. A proper dip-rack in a gym is more stable, but use what you've got. The beauty of the SEALS workouts are that you don't need a gym.
Dips are the equivalent of squats for the upper body. They work triceps, pectorals, shoulders, even back to some degree. If you can get to the point where you're doing 5 sets of 20 dips, you will have excellent useable and practical strength.
Peace,
Erik Y.