That's the housing for the draft spring. The 3 point lift has 2 modes, controlled by the small lever near the base of the large 3 point lift control handle. When the small lever is in the upper" position (pointing toward the front of the tractor), the system is in position control, and the large control handle controls the position of the lift arms. When the small lever is in the lower position (pointing down toward the ground) the system, is in draft mode, and the large handle controls how hard the lift arms will allow a plow or other ground engaging implement to "draft" , which basically means that at a certain amount of resistance, the lift will automatically lift up the implement to allow the tractor to continue forward in the event that the plow encounters a large rock or root that would otherwise cause the tractor to stop, or cause damage to the implement or to the tractor itself. The draft system has a spring as part of the automatic draft control and that spring is inside of that housing that looks like a bottle jack lying on its side.
