A satellite ground track is the line traced on Earth's surface by the point directly beneath the satellite (the sub-satellite point) as it orbits.
Because Earth rotates eastward beneath the orbit, each successive ground track is offset to the west. This is why a low-Earth-orbit satellite images different longitudes on each pass and only periodically repeats its coverage of a given area — the basis of revisit.
Pass prediction works by propagating the orbit to compute the ground track and then determining when that track (widened by the sensor swath and pointing agility) intersects an area of interest.
It is the point on Earth's surface directly below the satellite; the line it traces over time is the ground track.
Plan a real acquisition over your area on the interactive map, browse the satellite catalog, or read the tasking guides.