What does true to size mean in shoes?

I am selling a pair of shoes on ebay and the size is 8.5. The question I have received on them is: Are these shoes true to size? What does this mean and how do I answer it?What does true to size mean in shoes?
It means are the shoes a realy size 8.5...not a big fit nor a small fit.Answer honestly..wheter they are or not.What does true to size mean in shoes?
it means bigger than size or not bigger than size .