How does an improvised nuclear device pose a threat to global security?
Nuclear terrorism is largely a thing of theory and speculation. However, the easy access to plans and models, as well as the increasing availability of nuclear materials, makes the possibility of an improvised nuclear device more likely than ever before.
One issue with an IND is that it can be made small enough to carry -- "suitcase nukes" or "backpack nukes" -- while still having enough explosive power to destroy a large area. Because they can be so small, identifying and stopping them becomes harder.
Known thefts of highly enriched uranium have totaled less than six pounds or so. This is far less than required for an atomic explosion; for a crude bomb, over 100 pounds are required to produce a likely yield of one kiloton.
(Mueller, "The Atomic Terrorist?" cato.org)
While 100 pounds of uranium sounds like a lot, the density of uranium means that a smaller physical amount masses a great deal, making it possible for a fit person to carry it in a backpack with less effort.
Another significant aspect of an IND is the likelihood of it being a "dirty bomb," or a bomb that precipitates more radioactive material throughout and beyond its blast radius. This can kill far more people than the blast itself; modern nuclear weapons are very efficient, consuming most of the radioactive material in the blast, but a homemade IND is likely to be more inefficient, causing the death toll to be higher than a non-nuclear explosive.
Finally, if an IND is detonated in an act of terrorism, it is likely that it will start the classically theorized chain reaction of "Mutual Assured Destruction," where countries retaliate and react with nuclear weapons of their own. Such an outcome could decimate the human population on Earth, and could be started with one nuclear bombing of any major populated area.