Sunlight contains many kinds of energy: one is the visible light that we see, but it also contains infrared radiation (heat), a range of different kinds of ultraviolet (UV) radiation, radio waves, X-rays, and more. This is all real radiation, and it can burn you! The only reason why we're not all fried is that the amounts of the radiation are pretty small.

When you spend a long time in the sun, though, your skin starts to burn just like it would if you stood too close to a heat/infrared source or an X-ray machine. Our skin doesn't really play a role in that burning process--no more than a piece of paper plays a role in the burning process if you stick it in an oven.

Sunblocks don't change anything about our skin, they just contain chemicals that absorb different kinds of UV radiation. Since UV is kind of radiation that burns us the fastest in sunlight, sunblocks let us stay in sunlight longer without getting burned.