It is an introspective self-reported questionaire that is meant to determine your psychological preferences in how you view the world and make decisions. Constucted by Katharine Cook Briggs and Isabel Briggs Myers who based its off Carl Jung's conceptual theory. Consisting of four preferences, there are a total of 16 different types.
Find out your MBTI by taking the test provided by 16Personalities.
Note: The scientific backing on this is not strong but it still is fun to know.