Decarboxylation, or "decarb," is the chemical reaction that turns the acid forms of cannabinoids into their active forms. The best-known example is THCA becoming delta-9 THC. The reaction removes a carboxyl group (a carbon, two oxygens and a hydrogen) from the molecule, releasing carbon dioxide.
It happens with heat and, slowly, with time. When you light a joint, take a dab or use a vaporizer, the heat decarbs the cannabinoids almost instantly as you inhale. That is why smoking flower produces effects while eating raw flower does not do much.
Edible makers handle decarb during production, usually by heating the plant material or extract at a controlled temperature before mixing it into food or oil. Products like distillate are already decarboxylated, which is why they can be added directly to edibles or carts.
Decarb is also the reason lab labels use "total THC." The lab measures THCA and delta-9 THC separately, then estimates how much THC the product could deliver after full decarboxylation.
Too much heat or time can degrade THC into CBN and burn off terpenes, which is why low, controlled temperatures are generally preferred, both in production and in how many people choose to dab or vape.