Winterization of cannabis refers to the removal of fats, lipids, waxes and other unwanted or inactive compounds which is a critical step when purifying oils such as cannabis extracts. Winterization is necessary to create a refined, higher purity of product. Winterization involves using a nonpolar substance, a crude oil, and dissolving that into a polar solvent, like ethanol, at subzero temperatures.
This creates a mixture which is called miscella, that needs to be maintained at subzero temperatures for the compounds to coagulate so they can be filtered out and removed. (If you have ever stuck a pot roast in the fridge and pulled it out the next day to see a layer of fat coagulated on the top, separated from the meat, this is the winterization process). Winterization should be done prior to distillation and decarboxylation.