This means the file contains two distinct audio tracks that the user can switch between. Typically, this includes the original English audio track alongside a dubbed version in another language (such as Hindi, Spanish, or French, depending on the specific release uploader).
Based on the filename "the prestige 2006 x264 720p esub bluray dual au" , here are the likely of this specific media file: the prestige 2006 x264 720p esub bluray dual au