ok, the Unreal world is made from the geometry you create with the brushes --that's the BSP. It tells the engine where the flooor you walk on is, and when you bump into a wall.
Into this you place Actors -- bots, you, lights, sound, and decoration like the pipe.
The engine sometimes needs to know about actors touching each other. eg:
rocket hits player, player walks into trigger, player bumps a barrel.
Some actors, like lights, don't need to tell the engine about thing bumping into them.
To keep things simple on our primitive silicon-based computers of the Early Age of Mankind, this is done by giving each actor a Collision Cylinder. It's always a cylinder, and it's always upright. (the next version of the U engine apparently will offer more varied possibilities).
The size of the CC, and what sort of things it should notice touching it are all set in the Collision options.