What is this button on my MacBook? I have a 2011 MacBook. Along the left edge (looking at it from the normal typing position), near the front, there is a small silver button which is flush with the rest of the case. When pressed, a small series of tiny lights beside it flash green before going dark again. Nothing else happens.

Can anyone explain what this button does?
This is your battery capacity indicator. The number of lit dots indicates how much battery you have left.