THC is the molecule found exclusively in cannabis that provides the common effects of being “high” or “stoned.” THC triggers cannabinoid cell receptors in the body, giving certain effects. It was THC that was prohibited by law prior to legalization.
The amount of THC in a particular product is listed on the package in the form of a percentage. For example, a strain might have 20% THC or 30% THC. The higher the THC percentage, the more potent the weed is.