A new theorem predicts stationary black holes must have at least one light ring

Some actual details from the article once you get past the initial five paragraphs of breathless popscience-journalism:

> "In other words, we do not assume that one law of gravity is correct, but only assume that the correct law of gravity (i.e., whichever that is) allows the existence of black holes," Cunha and Herdeiro said. "Then, forcing the spacetime structure to obey to some regularity requirements, the existence of a black hole implies that there must be a light ring outside the horizon. In fact, a rotating black hole must have at least two light rings: one for light circling the black hole in the same spinning direction as the black hole and another one for light circling the black hole in the opposite direction."

What's a light ring from a time perspective? Seems as close to time travel as it gets, but that's just my pop intuition around timelike curves in spacetime. If it's a closed timelike curve outside the event horizon, is that a big deal for causality formalisms?

What does "stationary" mean in this context? There's no fixed reference point in the universe, so everything should be moving.