A transgender person is someone who identifies as a gender other than the one assigned to them at birth. A cisgender person is someone who identifies as the gender that was assigned to them at birth. A drag queen, or 'doing drag', is simply someone dressing up as a gender other than the one they identify as, but has nothing to do with their personal gender identity.
People have a difficult time understanding these concepts, Often times people assume that if a cisgender actor is portraying a different gender than the one they are in real life they are automatically either playing a drag queen or a transgender person. Because of this the film's producers went out of their way in press releases and promotion to clarify that the character of Susan is a cisgender woman who happens to be portrayed by a cisgender man.