what is catalytic converter?

what is catalytic converter?
This is the one after the the 4-1 header:

So now what is inside the Catalytic Convertor and why must remove?
For long Term usage all the dust and chemical will blocking on the Catcon,
and our vechile will getting Bad AirFlow and Hight fuel consumption from day by day Because of those Chemical&dust blocking inside the Cotton of Catcon



Usually in performance and fuel consumption wise,we would sugguest replace with direct flow center bullet or twister buller for making better airflow just like below