Ridings are a regional division in Britain, usually used to split up kingdoms.
The word Riding comes from Old English Thrithing and means "a third part" (similar in usage to the word farthing, which means "a fourth part.")
Ridings are often described by compass directions - i.e. North Ridings and South Ridings. The initial “th” sound got absorbed by the last sounds of North and South to make it into “riding.”