A magnetic field is the region around a magnet where another magnet or a piece of magnetic material feels a force. The force acts without contact. The field is strongest at the poles and gets weaker as you move further away from the magnet.
We show a field with field lines. Where the lines are closer together, the field is stronger. Where they are spread out, the field is weaker. The direction of the field at a point is the direction a north pole would be pushed if it were placed there, so field lines point from the north pole of a magnet to its south pole.
A plotting compass is a small magnet that is free to turn, so its needle lines up with the magnetic field at that point. To plot a field line, place the compass near the magnet and mark a dot at each end of the needle. Move the compass so that one end of the needle sits on the last dot, and mark again. Join the dots to draw the line. The north end of the needle shows the direction of the field.
