The dip is an exercise used in strength training. The person hangs from a dip bar with their arms straight, bends their arms to a 90 degree angle (thus lowering their body), and then slowly returns to their starting position of straight arms (lifting their body's weight). The difficulty of this exercise is increased by widening the distance that the person has to reach. Difficulty can be decreased by using an assisted-dip machine with a counterweight that helps push the body up.
Dips primarily train the triceps, and will also build the latissimi dorsi and pectorals with wide arm training.
Usually dips are done on a dip bar. Lifting the eintire body weight. To make the exercise heavier, you can hang weights onto your weight lifting belt. A lighter dip varitaion is "Bench Dips", where you do dips between two benches. Put your hands on one bench and you feet on another bench in front of you.