Question

what is magnetic field line and what are its characteristics?

Answer 1

the area covered by magnetic field is expressed in lines
under the area metals will attract

Answer 2

Magnetic field lines are formed around a magnet
The main characteristics of magnetic field lines are:
1. They are closed continues curves
2.They do not intersect each other
3.They comes from  north pole and goes to south pole
4.Inside the magnet it comes from south pole and goes to north pole