Question

what is defraction of light?

Answer 1

Defraction of light refers to the change in the path of light may be due to some significant heavy mass(blackhole) or due to slit.

Answer 2

When light waves find obstacles on their path, the bend around the corners of the obstacles. If they travel in straight lines, then one expects some shadow behind obstacles. But the bending of light wave and its falling in the expected shadow region is diffraction.
Due to this effect, there is an interference between light waves in that region and one may see regions of higher and lower intensities alternately - diffraction pattern.