The pan is a movement, a rotation of the camera about its position. Basically, it progressively changes the angle of view, without the camera moving. It is usually done with the aid of a tripod on which the camera will be fixed, even if some of the scenic are carried out in camera. The pan may be horizontal, following a movement from right to left (and vice versa) or vertical, in accordance with a movement from top to bottom (and vice versa).
The pan is usually easier to realize than the other movements in the sense that the camera remains in its position. It just performs a movement of rotation to give a global vision of a place that is not within the fixed frame of the camera, such as a landscape, or inside a fairly large size, or simply reveal a character or an object up then out of the frame. It is also used to follow a character or a vehicle is generally moving slowly through the scene. A vertical pan will be used more frequently to show the entirety of a high building, but use it to show a character from the bottom up, gradually, can have a value of a dramaturgic very strong, creating a certain tension in the viewer.
The pan is generally the most fluid as possible, without a sudden stop or change of pace during the movement.