The Navigation Bar is the bar at the top of the Pottermore website that allows a user to go to different parts of the website.
The Navigation Bar also has a link to the Gateway Story Map (via the Pottermore logo) and to the user's profile (via the picture of the house mascot). Above the spell's link there is an owl that gives users notifications such as friend requests.
Locked LinkThe far left link is locked (as of June 2012), with speculation that it may be used to go to Hogsmeade later in the website.
The html code name for the link, however, is "dockShop" (compared to dockDiagon, dockGringotts, dockHall, dockCommonRoom, dockSpells, dockPotions, dockTrunk, dockFriends and dockFavs), and the unlocked image, which can be seen in one of the two press release photos (available here) or directly (here) says "Shop Open", perhaps indicating that it was planned to link to the Pottermore Shop.