Conductors have electrons which are not bound tightly in their atoms. These are free to move within the conductor. However, there is no net transfer of electrons (charges) from one part of the conductor to the other in the absence of any applied electric field. The conductor is said to be in electrostatic equilibrium.
When a conductor placed in an external electric field E, the free electrons are accelerated in a direction opposite to that of the electric field. This results in build up of electrons on the surface ABCD of the conductor. The surface FGHK becomes positively charged because of removal of electrons. These charges (-ve on surface ABCD and +ve on surface FGHK) create their own fields, which are in a direction opposite to E.
It is a phenomenon of protecting a certain region of space from external electric fields. To protect delicate instruments from external electric fields, they are enclosed in hollow conductors. That is why in a thunder storm accompanied by lightning, it is safer to be inside a car or a bus than outside. The metallic body of the car or bus provides electrostatic shielding from lightning.