It is the geometrical construction to determine new position of a wave front at any later instant from its position at any earlier instant.
i. Every point on the primary wave front acts as a secondary source of light and sends out secondary waves (wavelets) in all possible directions.
ii. The new secondary wavelets are more effective in the forward direction only (i.e., direction of propagation of wave front).
iii. The resultant wave front at any position is given by the tangent to all the secondary wavelets at that instant.