It took me a while to figure this out, and only after digging into the official community help forum did it all make sense.
Stars are a thing of the past for Spotify, and the company is gradually phasing them out. "Plus" is the new star, and plussed songs fall into the songs collection under the new "Your Music" folder. Plussed albums are collected in the "Albums" folder in "Your Music."
This change may have been made to distinguish "Like" in radio, which influences future plays, from actually "Liking" and wanting to save a song or artist. (One guess.) Presumably, the old "Starred" playlist will soon disappear, or else fall below old playlists so as to hide from view.
To continue having easy access to these old "Starred" songs, it's recommended that you go into the folder, select all, and copy or drag and drop them into the new "Songs" under "Your Music." But don't expect to be able to sort by original date "starred" -- the new date added will overwrite the old metadata.
One major thing that differs the "starred" playlist and the "plussed" is that the plussed will not show up on your profile as the starred one. It's also not possible to set the plussed songs to be available offline.