Superclusters are clusters of clusters. In other words, large groups of smaller galaxy groups.
They're one of the largest structures of our universe, as well as other universes of similiar structure to ours.
Superclusters outside of universes are the same, but aren't limited to only containing cluster of galaxies. Superclusters containing verses in larger verses are common.
After Superclusters come Megaclusters, Gigaclusters, Teraclusters, Petaclusters, Exaclusters, Zettaclusters and Yottaclusters.