What is decarboxylation, in plain terms?

Heat converting THCA into THC. Fresh cannabis produces cannabinoids in acid forms — THCA rather than THC — and applying heat (smoking, vaporizing, oven-baking for edibles) strips a carboxyl group and converts the molecule into its intoxicating form. It is why "non-intoxicating THCA" and "gets you high when used" are both true statements about the same jar, why total-THC math multiplies THCA by 0.877 (the mass lost in conversion), and why claims built on the raw form deserve suspicion when the product is clearly meant to be heated.