Apple - What is this button on my MacBook?

This is your battery capacity indicator. The number of lit dots indicates how much battery you have left.

It's a battery capacity indicator, as noted in this Apple Support KB:

The battery indicator lights are located along the left side of the computer. This allows for quick and easy verification of the battery's charge status while the computer remains solidly in-place.

When you press the battery indicator light button, the corresponding flashes (or pattern of flashes) presents either the current charge status or a diagnostic status.

The KB also has a chart of the behavior shown when the indicator is pressed.

This allows to check the charge of your battery. When the battery is full all the green lights should flash.