Magnetic fields need to be blocked in some situations. For example, a hard drive shouldn't be exposed to magnetism. Military systems are often protected from electromagnetic pulses in the event of atomic detonations. Simple shielding can be accomplished with a plate of iron because the magnetic elements orient themselves in an external magnetic field to cancel it out. The north pole of the iron orients toward the south pole of the magnet, so anything on the other side of the iron plate will perceive a net magnetic field strength of zero.
Hold the magnet over the paper clip close enough that the paper clip starts rising toward the magnet a little.
Slide the iron plate in between the two, its plane oriented perpendicular to the line between the paper clip and magnet. The paper clip will drop away, no longer effected by the magnet.
Notice as you hold the iron plate that it is attracted to the magnet. The iron plate has itself become magnetized. Yet even though it's a magnet, it does not itself attract the paper clip as the permanent magnet did.
Perform the same shielding experiment with other materials, such as sheets of copper, aluminum, wood, rubber, glass or steel to see what will magnetically shield and what won't. Steel has high iron content and will shield, but none of the others will.