Objects in engines collide differently. Sometimes it’s as simple as two objects bump into each other on contact, but often there are cases in which one object has to pass through something, like a dropped weapon in Halo. Unreal’s tools allow easy access to change these principles and determine what you collide with, what you pass through, and what you don’t even see (like trigger volumes).
I’m poor at explaining things, hence why I don’t do these challenges often, but hopefully this described it well.