Need help identifying button on aluminum mac book?

[JillieSchmillie]

I have a 13-inch aluminum macbook. On the left side of the computer (the same side where you plug in the charger, headphones, etc) there is a tiny round button with a little row of lights by it. When you press the button, the lights turn green for a second. Nothing happens on my computer when I do this. What does this button do? What is it for? I've tried to google it and can't figure it out.

Thanks in advance!