When you click the name of the board, you can see, at the top of the message list for the board (over to the right) a count of the "followers" for the board. You can also click the link that says "follow board" - essentially this makes it a favorite within iHub for you and does two specific things - adds that board to the lists of boards shown in your iHub Favorites, and increments the counter.
Before they changed the terminology (to align with Twitter) this was a "board mark" and most here still use that term.
The number of board marks, therefore, is a metric indicating level of interest, and if that number rises quickly, as has happened here, it is a sign that the stock is "on the radar" for a lot more folks than before.