What does passable mean for a person with transsexualism?

To pass means that you blend as a member of the gender you believe you are. If you live as a woman, it means others accept you as a woman. Of course, "blending" is a more preferable term to "passing", since passing can imply negative things, such as counterfeiters passing fake money. Transition is about finally being who you are, not becoming what you are not.

The opposite of "passing" is "being read." That means someone can tell you were born with transsexualism. That is not desirable since the the whole point of transition is to live and be accepted 100% as if you were not born with transsexualism. A better way of saying you are being "read" is to say you are being "misread." That shifts the emphasis from your genitals of birth to the fact that others around you didn't accept you as you intended. Another alternative is to say that others "clocked" you.