THC stands for tetrahydrocannabinol — the primary chemical in cannabis responsible for its psychoactive effects. When THC enters the body, it binds to receptors in the brain and central nervous system, altering mood, perception, memory, coordination, and judgment. It is what makes marijuana a mind-altering substance.
There are multiple forms of THC.

The two you're most likely to encounter today are Delta-9 and Delta-8.