A magnetic field is the region around a magnet where a magnetic material, or another magnet, feels a force. You cannot see it, but you can map it with a plotting compass or with iron filings sprinkled on card.
Field lines show the field. They are drawn with arrows that point from the north pole to the south pole, because the direction of the field at a point is the direction of the force on a north pole placed there. The lines never cross.
The lines also show the strength of the field. Where the lines are close together the field is strong, and where they are spread out it is weak. Around a bar magnet the lines are closest together at the poles, so the field is strongest there. Moving away from the magnet the lines spread out, so the field gets weaker with distance.
Between two unlike poles that are close and face each other, the field lines are straight, parallel and equally spaced. This is a uniform field: it has the same strength and direction at every point in that region.
