Something affirmative expresses agreement or approval, like the affirmative nod your mother gives you when you ask to be excused from the dinner table. A formal way of saying "Yes" is "Affirmative!"
Anything affirmative is approving of something: affirmative words support someone or something. Being affirmative is the opposite of being negative or contradicting. A pat on the back is affirmative so is a signature you need to complete a form. Also, you can use affirmative as a formal way of saying yes to something. This is common in the military. It might sound a little odd, but if your teacher asked, "Does 10 plus 10 equal 20?" you could answer, "Affirmative!"