It's a massive concentration of major galaxy clusters, first noticed by Harlow Shapley in 1930, and hence the name
At a distance of 650 million light-years, in the Centaurus constellation, the Shapley Supercluster boasts of more than 25 clusters of galaxies containing some 8000 galaxies and with a total mass more than ten million billion times the mass of the Sun.
The Milky Way is part of a surge of galaxies moving past the Great Attractor, towards the Shapley Supercluster. In fact, it has been found that the Great Attractor and all the galaxies in our corner of the universe (including our Milky Way galaxy) are moving towards the Shapley Supercluster. This has led to current interest in this supercluster that it may indeed be one of the major causes of our galaxy's peculiar motion with respect to the Cosmic Microwave Background frame of reference.