An electromagnetic pulse has the ability to disrupt or disable devices and technology which rely on electrical power. When Earth was targeted by Anubis, the SGC used an electromagnetic pulse generator to counteract Anubis's Ancient weapon, which threatened to destroy Earth by building up a power overload within the stargate. Dr. Rodney McKay theorized that a massive E.M. pulse directed back through the stargate would destroy the weapon at the other end, however his idea proved unsuccessful, and the resulting power feedback instead shorted out all the SGC control systems.
Cross Reference: Anubis's Ancient Weapon, Electromagnetic Pulse, Rodney McKay
Episode Reference: Redemption