Generically, an extended body in free fall will experience deformation due to tidal effects. For instance, in a body falling towards Earth, those parts that are slightly closer to the Earth will experience a slightly stronger gravitational pull than parts which are further away. Some of the deformation will change the body’s volume. The shear is that part of the deformation which does not change the volume, only the body’s shape.
Some examples for shear can be found in the spotlight text Of singularities and breadmaking.