If you have not marked the board
you will see above a click-on that says
"Board Mark"

Also at the top of the page there is a link that
shows what boards you have marked -where
you can take back your board marks should you want to.

It is a courtesy showing interest and or appreciation of some kind.

It is the same as a person mark on your profile.
Where you have zero person marks

This board currently has 74 board marks which shows above (inc mine)