CBDa converts, or degrades, to CBD over time by breaking the carboxyl group from the compound. Heat & ultraviolet light speed the "decarb" process along; igniting (smoking) it for example would rapidly degrade the carboxyl group from CBDa creating CBD. THCa degrades to THC by applying heat, or naturally over time. Decarboxylation is referred to as "activation" as well because it makes the cannabinoids more readily accessible to absorption by our bodies.
You can easily decarb your own plant material in an oven at 190-220°F for 30-50 minutes. You can then infuse the cannabinoids into butter, oil, or any other fatty-acid-containing material, enjoy!