small button on the left-hand side of MacBook pro

I found a small circle button on the left-hand side of my MacBook Pro (late 2011) If I push that button, 8 green lights are turn on and then off. What is this button?

It's a way to check the battery charge level without having to wake the computer (or even turn it on). If it lights up in all green, then the battery is fully charged.

The newer models actually don't have that feature, but they also get 30 days of standby time now, so they probably figured it wasn't necessary anymore.

These are the battery indicator lights, which show the charge level of your battery when the button is pressed: