The phrase "ring plane" refers to the flat, disk-like region where the rings of a planet or object in space are located. It is like a horizontal plane where the rings form a circle around the planet/object. Full definition
The last ring plane crossing was a triple (it was seen three times over a few months), and occurred during 1995 to 1996. (science.howstuffworks.com)
The new belt lies at the far reaches of the Saturnian system, with an orbit tilted 27 degrees from the main ring plane. (aerospaceguide.net)
Paradoxically a «ring plane crossing» such as this gives astronomers a chance to unlock some of the secrets of the ring system and the moons that chaperone these lanes of ice and dust. (newscientist.com)