what does the green lights mean on the series 3 watch

Hi

Information about the heart rate sensor on Apple Watch, including the green LED lights and how they work, is detailed here:

Your heart rate. What it means, and where on Apple Watch you’ll find it. - Apple Support

The green lights are the constant pulse monitor. It has 2 low powere infrered lights you can’t see that take your pulse every few minutes and log back to the phone when it’s in range. The green ones are for contact monitoring for activity mostly and give and much more accurate reporting.

If you have the watch set to lock when it doesn’t detect it on your wrist it might be double checking you took the Watch off to lock it. Or maybe you were in a work out mode when you removed the Watch.

if your in a dark room, put it in work out mode or pull up the heart rate monitor and look at an angle under the Watch and you will see the same green lights when it actively reading your pulse.