A magnetic field is the region around a magnet where another magnet or a magnetic material feels a force. We draw it using field lines. Outside a bar magnet the lines leave the north pole, curve round and enter the south pole. The arrows on the lines show the direction a free north pole would be pushed, so they point from north to south.
The lines are closest together at the poles, so the field is strongest there. Far from the magnet the lines are further apart and the field is weaker. The more concentrated the lines, the stronger the field. Field lines never cross.
A uniform field has the same strength and direction everywhere. It is drawn as straight, parallel lines that are equally spaced. A uniform field is found in the gap between two flat magnets placed close together with opposite poles facing each other.
