The sun emits a particle known as a photon, which carries visible light to earth. These photons contain energy, which they radiate in the form of heat. When you put a magnifying glass in the paths of these photons, the convex lense of glass narrows and focuses the photons, making them converge on a narrower point then if the lense wasn't there. This concentration of photons concentrates the heat that they emit, heating the paper up until it ignites at a certain temperature (about 451 degrees F).