A pull-up is a kind of exercise that uses the weight of the body (body weight) to make a person stronger. To do this exercise one holds something that is high above the head with both hands so their body is hanging free under it. Bend your knees like your shins are parallel to the ground, it makes easier to do. Then one pulls oneself up and stops when the chin is above the hands. This mostly helps the back muscles but also uses the muscles on top of the arms (biceps) and stomach (abdominals). It is an exercise that does not need any special equipment except something high that a person can hold on to.
Many different ways to position the wrist and hands are used. Some say these are all different exercises, but they are all similar. Some kinds of pull-ups use weights on a belt to make the exercise harder.
Many who use bad form on this exercise will kick with their legs or jump and jerk with the legs to help the motion. Good form is holding the low parts of the body still and only using the arms to pull the body up.