The direction of magnetic field lines represented by the alignment of iron filings sprinkled on paper placed above a bar magnet

Compasses reveal the direction of the local magnetic field. As seen here, the magnetic field points towards a magnet's south pole and away from its north pole

A magnetic field line or magnetic flux line shows the direction of a magnet's force and the strength of a magnet.

The idea of lines of force was invented by Michael Faraday. His theory is that all of reality is made up of force itself. His theory predicts that electricity, light, and gravity have finite propagation delays. Einstein's theory agrees with this.