MBTI is a personality type. It stands for Myer Briggs Type Indicator. There are 16 personalities according to Myers Briggs test. They are considered to be one of the most accurate personality tests. Lots of writers use MBTI to help write characters that are very different from themselves and keep the character's personality consistent.
For example, I am INFP in real life, so it is easier for me to create characters with a similar personality.
There is tons of information online about MBTI if you're interested.