Sip

Sí, of course, means yes but what on earth does sip mean?

It turns out that as one-word responses, you'll often hear the casual forms, sip, for yes/yep, and nop, for no/nope. Simón (yes) and nel (no) are also heard.

Sí, sí, sí

Sí is sometimes said rapidly in a group of three: sí, sí, sí. This seems to fit when you're responding as right, good point. Remember, the store closes early today. Ah, sí, sí, sí. You're right, it does, thanks for reminding me of that. I've also heard Ah, cierto with a similar function.

You're welcome

If servers respond to gracias at all, it is often with a monotone sí rather than the expected de nada. And if someone thanks you for letting them borrow a chair from your table, simply respond: sí. It doesn't mean 'yes'. It's just a way to acknowledge their thanks and avoid responding with silence.

Be prepared to hear no meaning okay, I promise.
— No te tardes. Don't be late.
— No, ma. Okay, mom, I won't be late. (movie dialogue)

Okay

Surprisingly, sí also works for okay or to seek confirmation.

• Te hablo al rato, ¿sí? I'll call you later, okay? (movie dialogue)
• No lo hagas más difícil, ¿sí? Don't make things more difficult than necessary, okay?