Essentially, All the major nuclear powers have been part of a long standing agreement not to be the aggressors in armed conflict. Kind of like all the kids with sticks on the playground agreeing to never hit anyone else first.

Russia made up a bunch of false pretenses as an excuse to invade Ukraine, which violated those agreements, which is why no one like them right now. But, because Russia is a nuclear power, none of the other really powerful nations want to step in and directly help Ukraine. They are worried that Russia will view them stepping in directly as a declaration of war by another nuclear power, and use nuclear weapons in response. Russia has also threatened just exactly that.

However, despite the big show Russia is putting on, they are running out of troops, equipment, and ammunition, and anyone who supplies them to Russia will be sanctioned until their entire nation collapses.

That is where north Korea comes in. North Korea does not care about sanctions, because they are already completely sanctioned by the entire western world. They have a surplus of troops, because their entire male population is in their military. They also lack the food to properly feed their people, and have a giant stockpile of old soviet ammunition and weapons.

What appears to have happened is that Russia has taken some of those troops and material, in exchange for... we don't know exactly know what Russia gave or what North Korea gains, beyond speculation.

What we do know is that North Korea has operational nuclear missiles, which means that there are now two rogue nations on the battlefield in Ukraine.

This ups the ante quite a bit for the rest of the world.

Should we be worried about a nuclear attack?

No. The primary fear here is further escalation of conventional warfare, and those escalations triggering a full scale global conflict. Western nations haven't seen an actual war in over a generation, at least, not a serious one on the scale that wars used to happen.

There's also an undertone of uncertainty regarding what Russia gave North Korea, with logical guesses ranging from food and cash to ICBM schematics, which has been a long stated chief goal of the Kim regime.