A tank's cupola is a raised, armored structure on the roof of a tank/armored vehicle or tank turret that often carries observation devices and has a hatch.
Tanks of WW2 often had large cupolas on their turrets to allow space for periscopes or windows to give a crew member, often the commander, a much better view outside the vehicle. Usually they were also the most logical place for the commander's hatch, as it would allow them to stand partially outside the vehicle for a better view or to operate a top-mounted machine gun. As technology progressed and allowed for smaller periscopes and later cameras, cupolas became shorter and smaller, to the point that some modern vehicles don't have any at all.
