Welcome to the world of surf lingo. J In geology/oceanography you are correct a ground swell is cause by the ground moving. But in the surfing world a ground swell means a swell with a longer period (~10 seconds or more) which generally means better formed/cleaner waves. These swells tend to be created far away by storms and winds affecting a large "fetch" aka a large area of the Ocean and have time to organize well before landfall. The other type of swell is a wind swell which has a shorter period, and is generated by winds closer to shore and creates a choppier wave. It can be confusing lingo because both are created by wind.