The magnetic filed lines are the lines draw in a magnetic field along which a north magnetic pole would move.
The magnetic lines are also known as magnetic lines of force.
Activity to draw a magnetic field line outside a bar magnet from one pole to another pole. Take the magnet on a sheet of white paper fixed on a drawing board, using some adhesive material.
Mark the boundry of the magnet.
Place the compass near the pole of the magnet.
The south pole of the needle points toward the north pole of the magnet.
The north pole of the compass is directed away from the north pole of the magnet.
Mark the position of two ends of the needle.
Now move the needle to a new position such that its south that its south pole occupies the position previously occupied by its north pole.
In this way, proceed step by step till you reach the south pole of the magnet. Join the point marked on the paper by a smooth curve.
This curve represents a magnetic field line.