what does the symbol au mean?
Au also stands for Astronomical Unit. An Au is the mean distance from the Sun to the Earth and is used to state the distances to other planets. This way it keeps the numbers smaller to deal with.
The Astronomical Unit should be AU or au but not Au.
That's making the bad assumption that the person posting the question knew that, typed it correctly, etc.
It's reasonable to guess at what the question might have meant, but the answer should be correct in its usage.