Spikes are short duration voltage transients in an electrical current. Such transients may be caused by many things, including short circuits, tripped power breakers, lightning strikes, and AC malfunctions caused by your local power company. For sensitive electronics, excess current will flow through the circuitry if a voltage spike exceeds the equipment’s ability to handle it, often resulting in major damage. Many devices are now built with their own protection against spikes, while older designs, including guitar amps, may use a simple fuse which will “blow” and stop the current from flowing into the internal circuitry. This again points out the necessity of using power conditioners, particularly when plugging into older electrical systems, although it’s always best to play it safe and have a power conditioner for any of a number of electrical problems so that your equipment will not bear the brunt of voltage fluctuations.