When magnetic lines of flux encounter high permeability material, the magnetic forces are both absorbed by the material and redirected away from its target, much as a beavers dam absorbs and reroutes water from its den. The most effective shields are constructed as enclosures such as boxes or better yet, cylinders with end caps. Because the field follows the line of the enclosure, an enclosed shape keeps stray fields from finding gaps which could cause unintended interference.
Most magnetic fields are man made.
They can be found in solenoids, bar magnets, and some motors and transformers. In some cases, however, the fields interfere with sensitive electronic equipment, and shielding is necessary. Imagine a sponge. Some sponges have big holes, some are more tightly structured, each absorbs liquid to a point of saturation. Magnetic shielding material is similar.