Basically, it’s a loop that you can put your finger through to secure the phone when you’re holding it at weird angles (e.g., taking pictures from a high window, or through the guardrails of a bridge). Alternatively, you can use the ring itself as a sort of kickstand for the phone while taking photos on the timer or doing other handsfree stuff. It attaches to the phone/case by way of a sticker.

I haven’t purchased from any of these merchants at the following link, but you can see a bunch of examples here:

https://www.etsy.com/market/phone_ring_holder

Just be advised (at least in my experience) that these rings need to be replaced a couple of times a year (usually getting progressively looser). Because the holder can break at any moment, you should never rely 100% on the holder alone to keep your phone from falling. ;)