Magnetic field line
If there are a lot of lines through a magnet and not a great space between them, the magnet is strong. If the lines between a magnet are far apart and there aren't many lines, the magnet is weak. A way of determining the strength of a magnet is to do an experiment with iron filings. The iron filings will get attracted to the magnet and move into the shape of the flux lines. You then look at the shape of the iron filings and see the gap between the flux lines. This gives you an idea of the strength of the magnet.