This question is getting asked a lot since the release of iOS 9.3. The answer is pretty simple.
The star indicates a popular track – that is, one that folks choose to listen to a lot in the Music app. In iOS 9.3 the star replaces the popularity bar (for better or worse – I suppose we’d better get used to it).
On an album it’s likely the popular tracks with be the single releases, for example, or songs used on TV adverts.