The phrase "to get high" means to take drugs so you experience a change in your perception of the things around you. It's meant to be an enjoyable experience. The people in the joke are a passenger and a pilot. A pilot is someone that flies an airplane. The passenger asks the pilot "how high he would get" which most likely meant "at what altitude will we be at while flying?". The pilot misunderstood the question, thinking that the passenger was referring to sense perception. The joke is playing on the different meanings of the phrase "how high".
When asked by a passenger how high he would get , the pilot replied , ' I don ' t do drugs . '