Tarski's Theorem

I have just come across a reference to Tarski’s Theorem in a book I’m reading (Ken Wilber’s SES). I don’t know what that theorem says, so I have duly looked it up. Hence the link.

I still don’t understand it. Can someone explain it to me, please?

I’m seriously not that stupid in math. I’ve taken, and passed, Advanced Calculus. Twice.