A velarium ("curtain") was a type of awning used in Roman times. It stretched over the whole of the cavea, the seating area in amphitheaters to protect spectators from the sun. Precisely how the awning was supported is a matter of conjecture.
Retractable awnings were relatively common throughout the Roman Empire, including on the wooden amphitheater that preceded the Colosseum.
The Colosseum being the biggest amphitheater of roman times, the velarium that covered it was the biggest that ever was as well. It provided shade from the sun for up to one third of the arena. The velarium also created a ventilation updraft, creating circulation and a cool breeze.
In modern times