It is a giant, red, flower-shaped structure that is partially buried in the sand. It is located behind the buildings of Bikini Bottom. Its design matches the flower clouds that float above the city.
It was one of the original locations that Stephen Hillenburg designed for the show in his 1996 pitch bible. In the first concept map of Bikini Bottom, the flower arch was specifically labeled along with nine other locations. It was the only one that did not end up becoming an important setting in the series.