Mopping is a colloquial term for machine polishing cars which has now fallen into disuse but might still be used within the motor trade.

Before sponge polishing pads were more common, cars were typically polished using a lamb's wool buffing pads, which because of their shaggy appearance were nicknamed a 'bonnet' or 'mop'. 

Lamb's wool mops are still sometimes used because they can be harsh, especially when used with a coarse cutting compound at the beginning of the polishing process. When wet polishing, this could be a very quick technique for getting results fast, and so was popular in the trade when most cars didn't have clear-over-base style paintwork.