Its just normal but effective technique based on a simple concept. While User listening song one, start fetching song two from server so that use do not need to for buffering of song two. It increase the data availability and application seems very fast.
Dev byte publish a video talk about this concept. Let have an example of NewsReader.
Developer can download news text, title etc to show to the user on instant base and shows to user. The sooner you provide the content to user, the more he will feel .
See this video and get the depth of Pre fetching technique, find out where it works best.