the green light tells you if the joycon is turned on or not. the amount of lights on also tell you which player you are (player one has one light on their joycon/s player two has two, etc)
What are the lights on my switch controller?
As you may or may not already be aware, that ring is actually an LED light; we've seen it light up before but only when a Pro Controller connects to Steam. Starting from today, thanks to a Switch system update which takes the console's firmware to Version 9.0, the light will finally be used for "alarm notifications".10 sept 2019
How do I know if my Nintendo Switch Controllers are charged?
Step 1: For the Joy-Con charging dock, plug the included cable into the USB port on the Switch's dock. Step 2: Place up to four Joy-Con controllers into the dock and ensure the lights on the top are red. When they've turned green, the controllers are fully charged.13 dic 2021
Are all Nintendo switch controllers compatible?
All of Nintendo's controller features like HD Rumble, NFC for Amiibo scanning and motion controls are supported, and it works with or without wires. ... A number of cheaper third-party Switch Pro-like controllers are available, though.22 oct 2021