The Tech Stuff
Electromagnetic shielding is the practice of reducing the electromagnetic field in a space by blocking the field with barriers made of conductive or magnetic materials.
What It Means.
In order to keep accurate time, a precision mechanical movement will have to rely on all its moving parts to work at 100% efficiency and within all the design tolerances, which as you can imagine are tiny. So what happens when delicate metal moving parts come into contact with a magnetic field like the magnetic North of Earth?
Well, the forces can affect the small moving parts moving them out of alignment or tolerance, which in turn affects the movements ability to maintain accurate timekeeping. The answer is to shield the movement from the magnetic force, making the watch accurate regardless of location.
The Issue of magnetism was more of an issue in the past but with modern materials that are less affected by magnetic forces, this is now something of a complication rather than a necessary feature.