Yes is a confirmation or an affirmative — a positive reply. When you say yes to a friend's invitation to a party, it means you'll be there.
There's no more positive word in the English language: You use yes to agree, to confirm, or to answer positively. It works best of all to express uninhibited enthusiasm like when you get great news and yell it, maybe even closing your eyes in joy while you say it. Yes comes from the Old English gise, or so be it.