Big filter after the engine to collect all the black particular matter that comes out of a diesel. Every so often it starts to plug up with all the soot. The computer will then run a regen cycle. In this cycle it dumps extra fuel into the system. The filter gets wicked hot & all ate accumulated soot gets burned off leaving a nice clean free flowing filter.
More complex than the old systems with just an exhaust pipe, but a lot cleaner. So far I haven't seen any significant reports of issues. Any that have shown up have been taken care of under warranty.