What is a ridge - Hill climbing
The same answer you'd get on The Great Outdoors Stack Exchange: it's one of these.


Photo: ridge from Mount OtenSho to Mount Tsubakuro, Japan. By Alpsdrake; public domain; from Wikipedia

On a plateau, your value doesn't change much if you move in any direction. On a ridge, your value doesn't change much if you move in one direction, but it falls a lot if you move in the other directions.