Answer:
The strength of the field
Step-by-step explanation:
The density of field lines of a field (e.g. gravitational field, or electric field) indicates the strength or magnitude of the field itself. In particular:
- The regions in which the field lines are closer to each other (more dense) are the regions where the field is stronger
- The regions where the field lines are more spread apart (less dense) are the regions where the field is weaker
For example, if we consider the electric field generated by a positive point charge, the electric field is radial (outwards from the charge) and the spacing between the lines increases as we move away from the charge: this means that the strength of the field increases as we move away from the charge.