The reset button is a small button on your keyboard that's harder to reach than the rest of the keys. Often it's along the side of your keyboard, sometimes along the top edge, and on some keyboards it can be reached through a small hole in the bottom of the case. But what does it do?
The reset button causes the microcontroller on your keyboard to enter bootloader mode. That sounds rather technical, but it's really easy: pressing the reset button tells your keyboard to prepare itself for new incoming firmware. This is very useful, because it allows you to flash new firmware to your keyboard.