In flight, any rocket will rotate about its center of gravity, a point which is the average location of the mass of the rocket. We can define a three-dimensional coordinate system through the center of gravity with each axis of this coordinate system perpendicular to the other two axes. We can then define the orientation of the rocket by the amount of rotation of the parts of the rocket along these principal axes. The pitch axis is perpendicular to the rocket centerline. A pitch motion is an up or down movement of the nose of the rocket as shown in the animation.
The pitching motion is being caused by the deflection of the nozzle of the rocket. Such a rotation of the nozzle is called gimballing the nozzle and the motion causes the thrust vector to be inclined to the roll axis. Since the center of gravity is located on the roll axis, the thrust vector generates a torque about the center of gravity which causes the rocket to rotate. The Space Shuttle executes a pitch maneuver during its ascent into orbit.