In short, "Mr. Doctor". "Seu" is short for "o senhor" when used in conjunction with a name or other individualized expression, and especially in vocatives:
Seu Carlos, o senhor esqueceu a sua carteira.
Mas isso não foi falta, seu juiz.
Seu guarda, pode me informar onde fica a Rua da Anunciação?
Seu doutor, me dê licença pra minha história contar (Luiz Gonzaga)
"Doutor" is a popular form to address anyone with a college degree, or even just perceived as what Theon Greyjoy would call "our betters" - medical doctors (which in standard language should be "médicos", not "doutores") and lawyers, yes, but also engineers, professors, veterinarians, economists, etc. - and rich people, people dressed in a suit, people driving a car, well dressed white people in general, and so on.