What does the star next to track #2 and #3 indicate in this screenshot?
Looks like it indicates a song's popularity.
@rjonesy It's an indication of the song's popularity in the iTunes Store.
It means that either Apple has used this song, or, it means it is a popular track.
It's also the star rating that you've given a song/disc. While listening to a song, tap the artwork. From there you can give it 0-5 stars. The stars will show-up next to the song, like in your image.