A faraday bag is shielded enclosure made from metallic material that protects against the effects of electromagnetic energy.
Electromagnetic Shields can protect your electronics from exposure to electrostatic discharges produced from an electromagnetic pulse (EMP), solar flares or lightening strikes.
Why is it important to know about faraday bags?
If you can protect your electronics from, let's say, an EMP, you can continue to communicate or receive communications from others while those around you cannot. < In the case of an EMP, this effects a large region, so you may be the one person in your area who can continue to reach "the outside world" while normal communications are down. An EMP is so strong that a single detonation 200 to 400 miles over the center of the continental United States would fry every unprotected computer chip from coast to coast, and from the middle of Canada to the middle of Mexico.
Consider the damage from solar flares, too. Solar flares (sometimes also associated with a coronal mass ejection) are capable of extreme damage to modern computerized equipment! You can protect yourself to an extent by keeping a Hand Crank Radio on hand, but it even then, unless your electronic equipment is protected from an electromagnetic pulse, it will be fried! The solution is to have a faraday cage and keep your electronics inside when you anticipate something happening.