Toilet bowl rings are caused by either mold or hard water such as calcium buildup.
If the toilet bowl ring is a pale brown stain that look like rust it can be due to mineral deposits and hard water, while black, orange or green toilet bowl ring stains can be caused by mold.
You can get rid of toilet bowl rings by using some CLR to remove the calcium buildup and then scrub the toilet bowl and then flush.
Allow the CLR to sit in the toilet bowl on the toilet bowl ring stains for 20 to 30 minutes and then take a scrub brush to scrub the stain off.
If it's mold then using bleach would work better as the bleach kills the mold and also cleans the toilet as well.