A pan is a horizontal camera movement in which the camera moves left and right about a central axis. This is a swiveling movement, i.e. mounted in a fixed location on a tripod or shoulder, rather than a dolly-like movement in which the entire mounting system moves.
To create a smooth pan it's a good idea to practice the movement first. If you need to move or stretch your body during the move, it helps to position yourself so you end up in the more comfortable position. In other words you should become more comfortable as the move progresses rather than less comfortable.