An Asset contains all the track level information for each song on a release. This can be any and all of the audio file, the track title, the ISRC, the writers and performers for the song, and so on.
A Product defines the format of the release (single, EP, album, etc). The information here relates to the actual release and can include the title, the barcode, the catalogue number, and more.
You may wonder why you need to create an Asset and a Product for a one track single. All the DSPs receive and process information about releases and the individual tracks within them in the same manner and so this is industry standard. An Asset cannot contain the same information as a Product, so they need to be created differently, even if your release is just one track.