What is a pre catalytic converter?

[Unit 5302]

There are 4 catalytic converters on your car, but that's less than the 6 found on SN95 5.0s.

Anyway, from my understanding, the pre-cat's job is reduce and break up the nitrogen oxides in the exhaust, while the main cat's job is to oxidize the hydrocarbons and convert the carbon monoxide to carbon dioxide.

The pre-cat's essentialy make the job easier for the main catalytic converters.