Tetrahydrocannabinol (THC) is the active chemical substance in cannabis responsible for the potentially euphoric effects. So the larger percentage of THC, the stronger those feelings will be.
The potency (concentration or strength) of THC in cannabis is often shown as a percentage of THC by weight (or by volume of an oil). THC potency in dried cannabis has increased from an average of 3% in the 1980s to around 15% today. Some strains can have an average as high as 30% THC.
Cannabis that contains very low amounts of THC in its flowers and leaves (less than 0.3%) is classified as hemp.