The precise mechanism is beyond me, but suffice it to say that light is affected by gravity.

If you imagine throwing a ball in space in a straight line near a massive body (like a planet), the ball will curve and its new straight path will now be permanently deviated from its original straight line.

Now imagine instead of throwing a ball, you're emitting rays of light in all directions near a black hole. Light you emit towards the black hole will be lost to it, but light you emitted at an angle to the black hole will swing around it, just like the ball. If you imagine all the light you emitted slightly to the right, left, up, and down doing this, you can imagine that an observer on the other side could see all that light, appearing as though you were slightly right, left, up, and down from the black hole at the same time. This is what creates the ring.