The light on the back of the Circle Home Plus device is the LED Power Indicator.
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, open the app then Menu >> Settings and tap "Remove Circle Home Plus". This will remove the device and reset it so it's ready for set up again. Don't worry, you won't lose your profile settings after doing so.
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. You'll set it up by following this guide.
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.
No light: The Circle device is OFF. If this is not on purpose, you can troubleshoot power issues by trying another charger, charging cable, outlet, and holding the button for 10 seconds to try and power it on.