The Solar System's center of mass, including the sun, Earth, and all of the orbiting planets all revolve around this barycenter, and it's always shifting positions resulting from exactly where planets are positioned in their perpetual orbits.
Ephemerides, detailed maps showing the estimated positions of the Sun, moon, and all the planets over the course of a year, were one way to determine the solar system center and allowed for mariners to navigate by the stars. But these maps don't account for all the aberrations caused by anomalies like black holes' gravitational waves and planetary tugging.
NANOGrav harnesses the technology of massive radio telescopes like the installments at the Arecibo Observatory in Puerto Rico and the Green Bank Observatory in West Virginia, searching for variations in black hole disruptions and pulsars' beam time as they strike Earth caused by a slight warping effect of time-space ripples known as gravitational waves.