An "Alpha Strike" was a term used with large starships or ground vehicles with multiple weapons emplacements. The idea was to fire all weapons at the same spot, at the same time, to cause optimum damage. This could make a blast far more powerful then what any one weapon could do. An example of a ship prime for this tactic would be a Star Destroyer. Its angle, arrowhead design made the Alpha Strike perfect for the ship.
The reasoning was to destroy the target immediately, before it could fire, as to not prolong the battle. The only problem with this was that if the opening salvo didn't get past the opposition's shields, then the "alpha strike" ship would be defenseless for a period of time. The aggressive nature of the Galactic Empire made this tactic very popular amongst the Imperial fleets. Many others also employed this technique, such as the Yuuzhan Vong.