On the 5 panel tests we use at work, A is used as for amphetamines. I am prescribed Adderall, so it picks it up every time as a positive. I used to have to bring in my prescription to prove that I am not abusing or misusing drugs, but now they know what to expect. Different types of drug screenings have different abbreviations though. The best way to find out is to call and ask.
I assume it means positive for amphetamines but I don't know that. "A" could be like a reference number. Either way positive means positive. If you know it shouldn't have been then you need to look up anything that could cause a false positive. Sometimes you can ask for a retest but you will probably have to pay for it.