Question

What is magnetic field line ?in simple words

Answer 1

A magnetic field is the magnetic effect of electric currentsand magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude 
Magnetic fields can be produced by moving electric chargesand the intrinsic magnetic moments of elementary particles

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.