To "get high" means to feel really good. Usually it relates to drug use. To be "high on life" means that the person is enjoying life so much that it feels like they are on drugs. When someone is "High on marijuana", could mean they have been smoking funny cigarettes and are feeling pretty good about it, or it can also just refer to the person being intoxicated, without any reference to how they are feeling. "That person is drunk" - refers to alcohol. "That person is high" - refers to drugs.