Why are concave mirrors used in solar cookers?
They converge light to a single point (and so focus the heat there)
Another name for a concave mirror is a converging mirror, which pretty much sums up their purpose: point all the light that hits them on a singe point (the bit where all they rays actually cross each other is called the focal point). At this point, all the Infra Red radiation that has hit them (and been reflected by the mirror's surface) is focussed, and it is this IR radiation that actually does the heating up. Thus, the idea of a solar cooker is to put some food right on top of this focal point to greatly increase the amount of energy that is hitting it, and so heat it up and cook it.
Basically, they transfer all the energy that is hitting the large surface of the mirror to a very small spot.