Duolingo says redor by itself is around. It's that not true? If so what does redor mean alone?
"redor" refers to the surrounding area of an object. "ao redor" would literally mean "at the surrounding area". "redor" is rarely used by itself.
what is ao?
Around = ao redor
It's a contraction of "a o", meaning "to the" or "at the".
the dictionary dissagrees with you.