As a Maximum/Minimum Tilt
Tanks, Mechs, and other ground vehicles can tilt their turret or torso to a maximum pitch and a minimum pitch. Some tanks and mechs like Huits and Riflemen can look up to a near 90 degree angle. The minimum and maximum tilt can be found on each asset's wiki page under Torso pitch and Turret pitch.
Having a lower minimum pitch allows the asset to better shoot at targets at a lower elevation. This is most useful when shooting from a high vantage point and when shooting at BA that are in quite close.
Mechs can improve their maximum pitch by crouching using [right ctrl]. Minimum pitch is not improved by crouching.
As a Maneuvering Direction
VTOLs and Aerospace both aim and maneuver up and down using pitch. By aiming up with the [mouse] or the [space bar], aircraft can gain elevation, or if the aircraft is rolled perpendicular to the ground, it can instead make a sharp turn.
Aircraft have no maximum pitch or minimum pitch limitations like tanks or mechs.
For additional information on how to use pitch to maneuver an aircraft see the Air Combat Guide.