A magnet creates a disturbance in the space surrounding it called the magnetic field. For a long time, magnets were the only known source of this magnetic field.
The existence of the field can be revealed by a force that acts on certain materials and on charges in motion.
A simple experiment to perform would be to sprinkle iron filings in the neighborhood of the magnet. Each grain of metal becomes magnetized and, via mutual attraction, aligns itself along particular lines. These lines of force are also called field lines. They map out the magnetic distrurbance created by the sources present.