When two light waves of the same frequency and having 0 or constant phase difference travelling in the same direction superpose each other,the intensity in the region of superposition gets redistributed, becoming minimum at some points and maximum at other points.This is called interference of light.

Youngs double slit experiment-

In this experiment a source of monochromatic light illuminates a rectangular narrow slit S, this slit S is about 1 mm wide.Two narrow parallel slits S1 and S2 are arranged symmetrically and parallel to the slit S.An observation screen is placed at a distance of ≈ 2m from two slits.

Alternate bright and dark bands appear on the observation screen, these are called interference fringes.