Dual audio refers to a feature in video files where two audio tracks are embedded: one in the original language (in this case, Tamil) and another in a different language, often for dubbing purposes. This allows viewers who may not understand the original language to enjoy the movie in their native tongue. For Deiva Thirumagal, a dual audio download would typically include a Tamil audio track and another language, commonly English or a regional language, making the movie accessible to a wider audience.