Reclaiming is the act of breaking down an object and converting it to mass and/or energy for your economy. The most common target of reclamation is the wreckages that former units leave behind when they die. Live units can also be reclaimed, regardless of whose faction they belong to, with the only exception being Armored Command Units, which cannot be reclaimed, regardless of faction. In addition to units, certain ambient objects such as rocks and trees can be reclaimed as well, though their value is very low. The only units which can reclaim are Engineers, Armored Command Units, Support Armored Command Units, Engineering drones, the Cybran Experimental Megabot, and the Aeon T3 Heavy Assault Bot.
If you don't reclaim and the opponent is of similar ability, it is almost guaranteed you will lose
You can efficiently cut a way through walls by ordering any unit capable of reclaiming to go on a patrol that will lead it near or through the wall. This is because placing any unit capable of reclaiming on patrol will cause it to automatically reclaim any trees, rocks or wreckage within range. However, be warned that if you want to use this technique to remove enemy walls, combat units which are able to reclaim will prioritize fighting over reclaiming when enemy units are nearby. You'll have to clear away any enemy units near the wall before a patrolling combat unit will reclaim a hole in it. Reclaiming is also helpful in delaying the complete exhaustion of your mass supply (Mass Stall) if current expenditure exceeds production.