We set off a nuke with not enough U-235 or P-239 to go "boom"
Prudent to check that the triggers and all that other good stuff still work, so that when we drop a real one it has the desired effect.
Kabul weather: Today, sunny and hot. Tomorrow, scattered mushroom clouds with temperatures rising to 6000 degrees and winds from all around at 700-800 miles per hour.
Sub-critical means that the density in the "pit" doesn't go high enough to sustain a nuclear chain reaction. One way is to make it too small. Another is to use the wrong isotope. These tests allow you to evaluate the other components of the weapon without actually burning nuclear fuel.