Magnets create magnetic fields. These magnetic fields cannot be seen. They fill the space around a magnet where the magnetic forces work, and where they can attract or repel magnetic materials.
Although we cannot see magnetic fields, we can detect them using iron filings. The tiny pieces of iron line up in a magnetic field.
It would be difficult to draw the results from the sort of experiment seen in the photograph, so we draw simple magnetic field lines instead.
In the diagram, note that:
The magnetic field is strongest at the poles, where the field lines are most concentrated.
The magnetic field pattern when two magnets are used is shown in this diagram.
Note the different patterns seen when two like poles are used and two opposing poles are used.
When magnetic field lines are the same distance apart from each other, we say that the magnetic field is uniform. This is shown in the diagram: