What does the tick / check mark mean in the 'Recently added' album view?

When viewing an album in 'Recently added', I see this on the right-hand side of the window:

No 'tool tip' pops up when the pointer is over this tick / check mark... And clicking on it doesn't appear to do anything... And right-clicking doesn't give any options.

I remember the days when Apple would not have allowed such a spurious / undefined / unexplained user interface elements to be included in an app interface. At the very least include a 'tip' explaining its purpose when hovering over it!

Any clues?