From Mechwarrior Living Legends Wiki
To Dumb-fire a weapon means to fire a lock-capable weapon without acquiring any kind of lock, although the term can also be used to describe an unguided weapon. All missiles can be dumb-fired at any time, though in the default case they won't be guided towards a target.
By default, dumb-fired missiles fly along their usual trajectory towards wherever the targeting reticule is positioned at time of launch -- ArrowIVs and ground-based LRMs arc up, fly towards the target with some altitude, and descend when near target; all other missile types fly straight. However, a pilot has control over both SSRMs and MRMs when dumb-firing them: the missiles will turn to follow movement of the aiming reticule, giving the pilot the ability to guide the missiles towards a specific part of the target.
Dumb-firing is usually used for self-defense of Missile Boats: one can fire missiles low so that they arc up into a nearby opponent. Additionally, the missiles can be aimed at the ground, which can be an effective but difficult way to defend against nearby Battle Armors.
ArrowIVs and TBolts have an actual range longer than the one displayed, which in normal use allows them to hit fleeing targets. This disparity also allows for dumb-firing missiles at targets beyond a missile's maximum lock range. ArrowIVs, for example, can hit targets out to about 4000 meters, making them capable of reaching targets at enormous range, far beyond any missile lock-on. This tactic however requires the target to be stationary. Because of missiles' relatively slow speed they require a few seconds to reach distant targets (although TBolts and especially NARCs fly somewhat faster), so hitting a moving target is at the edge of impossibles, through it works well for at least one volley against unaware, stationary, long-range units, often including LRM Campers, AA, and Long Tom Artillery Tanks.
However, most missiles (with the exception of non-streak SRMs, MRMs and Fbombs) are capable of seeking towards NARC beacons and TAG lasers, but only when fired in active radar mode. When a pilot notices the spinning green TAG marker or pulsing yellow NARC marker on its radar within the missile range, he can dumb-fire the missiles in the direction of the target, and they will be guided to the target as long as the indicator is active. Missiles will also try to seek towards a the NARC or TAG if it appeared after they were fired, or once they close to within 1000m of an active NARC or TAG. This method is often used with Arrow4s: one pilot dumb-fires Arrow4s while an allied player paints a target with a TAG laser. With a high level of coordination, such as may be found in a clan, the TAG player can delay painting the target until the missiles are close. This delay can help protect the TAG player from detection or hostile fire. However, without tight coordination it may be more effective to TAG the target continuously and simply hope not to be noticed.