As a flight instructor you gain experience by teaching others to fly. This way you learn the material extremely well (learning by teaching) and you get paid to fly. This is a very common way to start a flying career in the United States.
The instructor licenses come with ratings limiting you to certain areas of instruction. A regular instructor for instance can only instruct private pilot and commercial pilot students on single engine aircraft. To do instrument instruction an instrument instructor rating (CFII) is needed. Further to train students in multi engine aircrafts you need a multi engine instructor certificate (MEI).