The astro-body could be free falling directly to the gravitational field source, but in many actual cases the astro-body is orbiting the center of mass of celestial frame consisting of a system astro-bodies (to which the first astro-body belongs too) and that system (excluding the first astro-body) is collectively the gravitational field source. The simplest case is a gravitationally-bound two-body system.

In brief, the tidal force is a stretching force due to variation in the gravitational field.