What do the letters associated with the MBTI instrument mean?

The MBTI instrument has four sets of letters:

E and I stand for Extraversion and Introversion—indicating whether you get energy from being around people or from time spent alone.

S and N stand for Sensing and Intuition—indicating whether you become aware of specific facts and concrete details or prefer to focus on hunches and the big picture.

T and F stand for Thinking and Feeling—indicating whether you tend to make decisions based on logical analysis and the principles involved or prefer to decide by considering your values and promoting harmony for the people involved.

J and P stand for Judging and Perceiving—indicating whether you prefer your life to be planned and like it when things are decided or prefer to go with the flow and like keeping your options open.