[wiml]

It means that the black hole is close to extremal. This means that the space near them is deformed in even more interesting ways than around a boring old non-spinning spherically-symmetrical hole.

I think it's often assumed that most black holes in the universe would have relatively low spin, compared to their mass. The research here is saying that, perhaps, the way that stars collapse would produce near-extremal holes in most cases, so they would be common in the universe.

I recommend reading up on black hole geometry ad Kerr(-Newman) holes; it's likely that any description that fits in a HN comment will leave you knowing less than when you started.