What is the Great Attractor?
A gravitational anomaly
The Great Attractor is an apparent gravitational anomaly at the center of the Laniakea Supercluster, the supercluster the Milky Way exists in. Copied and reworded from Wikipedia
The Great Attractor is called an "apparent gravitational anomaly" due to the fact it lies behind the Zone of Avoidance, the center of the Milky Way Galaxy. Therefore the extreme brightness of light obscures the view.
The anomaly has an apparent mass thousands of times of the Milky Way Galaxy that is concentrated in a much smaller space.
We are about 250 million lightyears away from the Great Attractor and is currently moving at about 370 miles per second towards it.