After you’ve created an account, you can favorite tracks to quickly save them for later or archive them to your own created collections.
To favorite a track: click on the star-symbol in the track player.
To add a track to your collections: click on the plus-symbol in the track player. You can choose to add the track to a previously created collection or to create a new collection for which to add the track.
You can find your created collections, or create a new one, under "Your Collections" on the main page.