A Yes/No question is a question that can be answered with yes or no. Easy, isn't it? Hold your horses and don't leave just yet. I'm going to tell you something that will make you sound much more natural the next time you speak Portuguese.
One of the characteristics of this language is that you don't need to say yes or no. Well, you can if you want, but it's not the only way to answer this type of questions. In fact, if you want to show a person that you are really interested in what he or she is saying, you can use the verb repetition construction. What's that? Let's see a couple of examples:
Question: Você quer vir comigo ao cinema?
Answer: Sim / não.
Question: Você quer vir comigo ao cinema?
Answer: Quero!
Question: Você gosta de viajar?
Answer: Sim / não.
Question: Você gosta de viajar?
Answer: Gosto! / gosto muito!
Can you see the difference? By repeating the verb of the question, you are demonstrating that you have paid full attention and that you are especially interested in what the other person is talking about.