A stillage is like a pallet or skid, but with a cage or sides, or some form of support specifically tailored to the material it is intended to carry.
Unlike kegs, which can be simply stood upright on the floor, casks are used lying on their sides.
This allows the beer to run from the tap under gravity, with room in the "belly" of the cask below the outlet for the finings to collect.
It is important that the stillage holds the cask absolutely still with no rocking or shaking; otherwise, the sediment will be shaken into suspension and the beer will be cloudy.
At temporary events, sturdy tables or frames made of scaffolding and planks might be used, with the casks placed on wooden wedges (two at the front, one at the back).