Coherence is attained when there is a constant phase difference in two or more waves over time.
Two waves are said to be in phase if their crests and troughs meet at the same place at the same time, and the waves are out of phase if the crests of one meet the troughs of another.
The light produced by lasers is coherent light.
Incoherent light contains many different wavelengths due to which it is scattered. An example is the light from fire which emerges in every direction.