A letter from a trained professional eg psychiatrist, psychologist or counsellor that enables you to start T (testosterone). It means you've attended sessions, talked about your issues and usually have been diagnosed GID (Gender Identity Disorder) or suffering from Gender Dysphoria according to the HBS (Harry Benjamin Standards of care) and/or the DSM (Diagnostic and Statistical Manual of Mental Disorders).
Though IMHO I dont have a disorder - I have a birth defect. I'm sure lots of other guys and girls agree there!