A ships' General ARea Defense Integration Anti-spacecraft Network (GARDIAN) consists of anti-missile/anti-fighter laser turrets on the exterior hull. Because these are under computer control, the gunnery control officer needs to do little beyond turn the system on and designate targets as hostile.
Since lasers move at light speed, they cannot be dodged by anything moving at non-relativistic speeds. Unless the beam is aimed poorly, it will always hit its target. In the early stages of a battle, the GARDIAN fire is 100% accurate. It is not 100% lethal, but it doesn't have to be. Damaged fighters must break off for repairs.
Lasers are limited by diffraction. The beams "spread out", decreasing the energy density (watts per m2) the weapon can place on a target. Any high-powered laser is a short-ranged weapon.
GARDIAN networks have another limitation: heat. Weapons-grade lasers require "cool-down" time, during which heat is transferred to sinks or radiators. As lasers fire, heat builds within them, reducing damage, range, and accuracy.
Fighters attack in swarms. The first few WILL be hit by GARDIAN, but as the battle continues, the effects of laser overheat allow the attacks to press ever closer to the ship. Constant use will burn out the laser.
GARDIAN lasers typically operate in infrared frequencies. Shorter frequencies would offer superior stopping power and range, but degradation of focal arrays and mirrors would make them expensive to maintain, and most prefer mechanical reliability over leading-edge performance where lives are concerned. Salarians, however, use near-ultraviolet frequency lasers with six times the range, believing that having additional time to shoot down incoming missiles is more important.
Lasers are not blocked by the kinetic barriers of capital ships. However, the range of lasers limits their use to rare "knife fight"-range ship-to-ship combat.