Pitch
Pitch has two main meanings in MWLL. Both meanings typically involve using the [mouse] Y-axis, up and down. Movement along the mouse's X-axis is called yaw.
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 higher maximum pitch allows the asset to better shoot at targets at a higher elevation, especially Aerospace and VTOLs.
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.
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.