Gravitational Lines of forces are imaginary lines in the
region of gravitational field which gives information about the net
gravitational field in the region at any point. The information includes the
direction of gravitational field vector as well as the idea about strength of
the gravitational field strength.

At any point in a region of gravitational field the tangent
to a gravitational line of force gives the direction of field vector at this
point and the density of gravitational lines in the neighborhood of that point
gives the idea about the strength of the field. If in a region lines are
closely spaced or denser than in the close neighborhood of this point
gravitational field strength is considered to be more compared to a region
where line density is less.

See the below video for the detailed explanation –