Question

what are magnetic fieldlines?roughly of magnetic field lines for a bar magnet

Answer 1

the lines which are being pass in a magnet through which a northpole could move

Answer 2

The magnetic field can be defined as:

“A region around/surrounded by a moving electric charge or a magnetic material within which the force of magnetism acts.”

How does Magnetic Field occur?

If you are wondering how Magnetic fields occur. Well, it occurs whenever a charge is in motion. That being said if you apply more charge in the motion the strength of a magnetic field usually increases. Besides, Magnetism and magnetic fields are part of the electromagnetic force which is indeed one of the four prime forces of nature.