Whenever someone posts in a board (let's use General Board for example) the falcon on the glove turns white showing that there are new posts in that board that you have not read. If you have read the posts in the board, the falcon is a white outline with a grey center.
When you click "Mark As Read" it will change that falcon from white to white-outlined grey.
It's just a feature for those people who want to keep what they've read and haven't read organized.
