Slipping is moving your head and/or torso out of the way of a punch. It is best to slip toward the side the punch is coming from, either by sliding the head toward that direction or leaning from the waist either backward or to the sides. This way, you are out of range of a follow-up punch with the other hand. Do not dip the head forward of lean forward; that is just begging to get hit.
Slipping only involves a slight movement; you want the punch to miss just barely. The reason for using only a slight movement is so you may conserve energy and stay in position for a counterattack. Slipping to the outside makes it more difficult for the opponent to make a follow-up attack; however, there will be times you want to slip to the inside of a punch, such as slipping to the inside of the jab and then countering with a hook to the body.
Some slip by rocking the upper body at a 45-degree angle to the left or right of a punch while leaning slightly forward. Some add a step to the slip to help setup for a counterattack.