When the light ray incident on a glass slab, there are two refraction.

Firstly When ray incident on the slab,it enters in glass slab(denser medium) from air(rare medium).

Secondly when light coming out in air from glass slab.

In this, the final emergent ray is parallel to the incident ray but laterally displaced with respect to the incident ray.

This shift in the path of light onemerging from a refracting medium with parallel faces is called lateral displacement.