Magnetic lines of force of a magnet shows the direction in which a magnetic material will be aligned when placed at the location. Magnetic field lines for a bar magnet are as shown in the diagram.
definition
Arrangement of iron fillings around a bar magnet
When iron filings are placed in magnetic field, they are magnetized and induce poles in them. Hence, they act as small magnets themselves. These filings orient themselves along the magnetic field lines of the bar magnet as shown in the figure.
