When a laser beam is emitted from a source, there may be phase difference between the beams at different points that is it may be possible that there may be some time gap between the two atoms to come from upper energy level to lower energy level. The term coherence refers to the degree of co-relation between the phases.
In other words, two or more light waves are said to be coherent if they bear a constant phase relation among themselves.
Reference: This article is referred from my authored book “optics and lasers” having ISBN 81-272-2948-2. In case of any doubt, post in the comment section.