Just like in your science basic, a lightning bolt is used to represent a source of electricity or electric shock hazard. In most phones, you see a lightning bolt on your battery when a charger is connected to it.
Apple has designed their devices to give off their best in all situations. As the Apple Watch is basically a time-telling machine.
It is just reasonable to make sure it tells time always. With this intention, Apple designed the battery reserve mode on the Apple watch to keep your Apple watch active for a longer time when the battery goes low.
Why is my Apple Watch showing a red lightning bolt?
When you see a red lightning bolt on the device, it means your device is in the power reserve mode. When this happens, your device is disconnected from any paired devices. More also, you will not be able to use your iPhone features in this mode.
This is to make the Apple watch keep the time for you until you are able to reach a power source.
To be able to turn off this mode, you need to charge your Apple watch. Connect your Apple watch to its’ charger. Once the battery is full, remove it from charge and press and hold the button to restart the watch.
Once your watch restarts, you can be able to connect with your iPhone and other devices and use your Apple watch normally
Note that power reserve mode cannot be excited when the watch is still on charge or the battery is below 10%.