This is my toilet waste pipe, I cut a little hole in the dry wall and used a web-camera to take this picture:
Close up with the leak:
Looks like it's leaking from the joint between the rings, I can visually see the leak every time I flush the toilet upstairs. I'm new to plumbing and not sure what are these 3 plastic rings for, and the main question - how do I fix this leak?
The "rings" are coupling ends. You need to cut out the leaking section and replace it. Most leaks from fitting joints are merely bad glue mistakes,but need to be replaced since there is no repairing them.
If you have never made a mistake, you haven't done much.