Wow, so much that is that is not known.
What they are doing is a missile hitting (actually touching) another missile. This is not a bullet hitting a bullet which is simpler, a bullet travels about 2500 Feet Per Second (fps), the ICBM travels almost ten times that at 22,000 fps.
The difference between a rocket and missile is that a missile is guided (that means corrects its course in flight), a rocket is ballistic subject to aerodynamics.
Now if you can achieve a missile to missile engagement that keeps effects outside the blast area, you are successful.
This is a non trivial problem, this successful test will lead to more success.
“the ICBM travels almost ten times that at 22,000 fps.”
Which means shrapnel from an ABM hits it at that speed, ripping holes in it.