What is the ozone layer made of?

Relatively high concentrations of ozone. This layer absorbs 93-99% of the suns UV light, which is potentially harming to life on earth.

The ozone layer is made of the usual constituents of atmosphere, ~78% nitrogen, ~21% oxygen, a bit of argon, and way down on the list is ozone at about 9 ppm (rather than ~0.05 ppm like is found at Earth's surface). The ozone layer does not contain as much water vapor as the lower atmosphere does, which helps ozone stay longer.

An ozone is composed of three oxygen atoms (O3) rather than the usual two (O2).