"Dual Audio" means the video file contains at least two different audio tracks. This is a crucial feature for viewers who prefer to watch a show in their native language or a language they are learning. For Stranger Things , the original audio is . A dual audio file would likely also include a dubbed Spanish track, as official releases often do.
This means the video file contains two separate audio tracks that you can toggle between using your media player. For viewers in non-English speaking regions, this usually includes the original English audio track alongside a localized dubbed track (such as Hindi, Spanish, or Portuguese).