MTF stands for 'male to female'. It describes a transgender woman — a person who was called a boy at birth but whose real gender is female, and who has moved, or is moving, towards living as a woman. That move can include medical steps (hormones, surgery) but does not have to; the letters are about the direction of the change, not about any particular operation. The matching term for the other direction is FTM, female to male. You mostly meet MTF in writing — medical papers, forms, older online communities — rather than in daily conversation. It is important to know that many transgender people today prefer not to be labelled this way. The letters put the birth assignment first, which can feel like saying 'she used to be a man', and that is not how most trans women see their own lives. The usual, safer thing to say now is simply 'a transgender woman', or just 'a woman'. So learn MTF so you can read it, but in your own speech, use the plainer words unless the person you are talking about uses MTF for themselves.