Flight Rings can be found all over The Abyss. When flying through a Flight Ring your Flight Meter will be part way replenished. Usually the most the Flight Ring will give you is +7 Seconds back on your Flight Meter. Also the Flight Rings have a built in "hidden" cooldown of about 3 seconds. Which means if you fly through it and then fly back through it within that time your meter won't be Replenished twice.
Even though the Flight Rings have a hidden cooldown it doesn't mean that you can't keep flying through the same Flight Ring TO replenish your flight time. It is very common to see players flying through the ring for a few minutes to replenish their Flight Time. This strategy can be seen in the video below:
Picture of a Flight Ring: