Graceful degradation takes the opposite starting point from progressive enhancement: build the complete, richly interactive experience first, then make sure that if something is not supported, an older browser, a particular assistive technology, a slow connection, the experience falls back to something reduced but still functional rather than breaking outright.
In practice, most accessibility-mature teams blend both approaches, favoring progressive enhancement as the default building strategy while applying graceful degradation thinking to specific complex features that genuinely need a rich baseline experience to be useful at all.