What is meant channel length modulation in NMOS transistor?

The current between drain and source terminals is constant and independent of the applied voltage over the terminals. This is not entirely correct. The effective length of the conductive channel is actually modulated by the applied VDS, increasing VDS causes the depletion region at the drain junction to grow, reducing the length of effective channel. The increase of depletion layer width at the drain as the drain voltage is increased. This lead to a shorter channel length and an increased drain current is called channel length modulation.