The light on the back of the Circle device is the LED Power Indicator, as demonstrated in the following diagram:
Depending on the light's pattern, it can indicate the following:
Solid light: The Circle device is ON and paired to a network. If you need to unpair it, see this article.
Slow blinking light: The Circle device is ON but unpaired; it either cannot see the wireless network it was paired with, or it has yet to be paired to a network. See this article for troubleshooting steps.
Fast blinking light: The Circle device is ON, but it's in the process of factory resetting, or there is an internal issue (like a low battery). Make sure it is plugged into power via USB, and let a few minutes elapse. If the light is still blinking fast, please contact us using the links below.
No light: The Circle device is OFF. If this is not on purpose, see this article for troubleshooting steps.