What is the icon and number next to the name of a board on the board drop down? I would guess it has something to do with new activity. The "Trello Development" board, for instance, had a 100 next to the little icon. But there certainly aren't 100 new cards.
The icon for the activity stream is the same icon as the one in the drop down, so the number represents new entries in the activity stream. Would suggest adding "Activity" to the Glossary in the help section.