It is the present subjunctive of the verb to be, the second-person singular form.

Another way to explain it... (In future) Imagine you have a friend who is studying to be a doctor. As a good friend, you expect that >to become< a good doctor. In spanish, you can say: > Espero que llegues a ser un buen doctor. (or) > Espero que seas un buen doctor. Now, imagine that your friend and became a doctor and come to assist you. As a friend, and patient, you expect him to be a good doctor. In Spanish, you can say: > Espero que él sea un buen doctor.

its like i hope `you to be` a doctor. : it is be in future :)