Simply put, THC is what gets you high when you ingest cannabis. THC stands for Tetrahydrocannabinol and is only one of many Cannabinoids that can be found within cannabis.
THC is actually found on the plant as THC-A, which would not get you high. This is why you would not get high if you ate raw Cannabis. THC-A is converted to THC through heat, which is usually done by your lighter when you smoke or by a vaporizer. This is why when making edibles, one must decarb as well.
THC, while psychoactive, has been found to be beneficial to one's health as well. Common uses of THC include the treatment of Nausea and Pain Relief, and THC is also an Anti-Inflammatory, Anti-Oxidant, and Anti-Bacterial.