Atrac is an obsolete Sony format. It's what I used to burn stuff to for my old Sony MP3 player. They've stopped supporting it, although I'm sure a number of their products will still play it. The format was OK, it was better than MP3. However MP4 or WMA Pro will give you as good or better sound quality, imho.
There's a software tool from Sony that will convert Atrac to MP3. They released it when they stopped supporting Atrac. You'd probably have to search sony support sites or google to find it.