Short Story: You need to cook your cannabis to “activate” the necessary cannabinoids. Decarboxylation is the changing of cannabinoids. Examples include THCA to THC and CBDA to CBD.
Long Story: Decarboxylation is a chemical reaction where a carboxyl (COOH) group is removed from certain cannabinoids changing them into the cannabinoids most discussed in today’s literature (THC, CBD, CBG, etc). It is these cannabinoids that interact with your endocannabinoid system. In cannabis this happens naturally as a part of the harvesting, drying, and curing process. It is actually a process of decomposition, or breaking apart. Heat is used to speed up this process. That is why cannabis is smoked-to immediately decarb the flower. In other products, such as tinctures, butters, oils, and edibles, decarboxylation, or heating, must be done before, during, or after the resins are extracted.
For most people, the cannabinoids present AFTER heating are more bioavailable-meaning able to be used by your body. There are some who are benefiting from a product that is not decarbed and has the chemical make up of mostly acid cannabinoids. See Where to Begin or Cannaibinoid Life Cycle-THCA to THC. Note that if you are using, or wish to use, a product for its acid compounds, they will slowly breakdown into their counterparts over time.
There is much debate about what temperature to decarboxylate and how long. Most of us will want to fully decarb the flower. Remember we are trying to mimic nature. Slower is better. There are many variables that impact the cannabinoid composition. How long has the weed been harvested? How has it been stored? What cannabinoids would I like to see present in the final product? Even flower that has been curing for over a year is still going to benefit from decarboxylation. For beginners, just cook your cannabis in an oven, or toaster oven, at 240f/115c for 45 minutes. If you scorch the weed it is still usable but you may have lost some cannabinoids. Gentle is the strategy.