This is a significant development, and potentially scary, for several reasons.
Maybe first and foremost, it shows that North Korea's nuclear program might just be sanctions-proof. The United Nations and United States sanction North Korea's economy for a bunch of reasons – to punish human rights abuses, to deter the seemingly random acts of military aggression – but one aim is to make it really hard for the country to import the sophisticated technology necessary for its nuclear weapons program. Now that North Korea may have developed that technology at home – something that requires all sorts of advances in not just enrichment but in iron- and steel-working as well – it doesn't really matter if we're blocking their ability to import specialized centrifuge lathes. North Korean leader Kim Jong Un can walk right up to the de-militarized zone and thumb his nose at our trade blockade.
One big effect of this development is that North Korea's nuclear program is far less constrained than it was before it made this big step toward nuclear self-sufficiency. Don't worry too much, its program still falls far short of every other nuclear weapons program in the world by every measure – and its missiles can't reach very far anyway. Still, the North Korean nuclear program can grow a lot faster with self-sufficient technology – and there's a lot less that we can do to stop that.
This also adds more evidence to something that's been suspected for a while: that North Korea is transitioning from plutonium warheads to uranium. Uranium enrichment is easier to hide, which means it can enrich more without us stopping them or even knowing the size of its arsenal. Uranium is easier to get in big quantities – plutonium has to be produced in a plant, uranium you just dig out of the ground – which means the country can make far more warheads. Weapons-grade uranium and uranium enrichment technology is easier to ship abroad than plutonium, which worsens the threat that North Korea could export to other rogue would-be nuclear states.
The risk of North Korea proliferating its nuclear program abroad would seem to have increased at a sensitive moment: just as Iran, presumably a potential recipient of North Korean technology and know-how, signals its willingness to talk with the West about settling the nuclear issue. The stakes of those nuclear negotiations between Iran and the West would seem to have just gone up a little bit. North Korean nuclear advances are also potential Iranian nuclear advances – bringing Tehran a touch closer to break-out capability.