THCA, short for tetrahydrocannabinolic acid, is the form THC takes inside a living or freshly harvested cannabis plant. On its own, THCA is not intoxicating. Eating a raw bud will not produce much of a high, because most of the plant's potential THC is still locked up as THCA.
Heat changes that. When you smoke, vape or dab, or when you bake flower for edibles, THCA loses a small piece of its molecule (a carboxyl group) and becomes delta-9 THC. This process is called decarboxylation, or "decarb."
That is why lab reports for flower often show a high THCA number and a small delta-9 THC number. Labels usually combine them into "total THC," which estimates how much THC the product can deliver once heated. The formula is THCA × 0.877 + delta-9 THC. The 0.877 accounts for the weight lost when the carboxyl group comes off.
You may also see "THCA diamonds," which are crystals of nearly pure THCA grown from a concentrate. They become very potent once dabbed. When you compare flower, compare total THC rather than THCA alone, and remember potency is only part of the picture. Terpenes and freshness matter too.