These are the curve lines around a magnetic field from north to south pole.
The curved lines along which the iron fillings align themselves or the path along which the freely pivoted magnetic needle moves are called the magnetic field lines of force.

Magnetic field line are those imaginary line which shows the direction and strength of magnetic field. It emerge from North pole and merge at south Pole. It forms a close circular loop.
The path along which north pole moves in a magnetic field is called magnetic lines of force or magnetic field line. They show the direction and strength of magnetic field. If magnetic field lines are very close to each other in a particular region,then the strength of magnetic field in that region,is very large.

A field line is a locus that is defined by a vector field and a starting location within the field. Field lines are useful for visualizing vector fields, which are otherwise hard to depict.A convenient method to describe the magnetic field around a magnet is to draw magnetic field lines around it. Magnetic lines of force are the imaginary lines introduced by Michel Faraday to visualize the magnetic field.Magnetic field lines were introduced by Michael Faraday (1791-1867) who named them ‘Lines of force’. The lines of force of a field are a very useful concept in physics. Lines of force are a graphical representation of a field. The line of force is the path along which an isolated unit North Pole would move in a magnetic field.The lines that we have mapped out around the magnet, called the magnetic lines of force, indicate the region in which the force of the magnet can be detected. This region is called the magnetic field. If an iron object is near a magnet, but is not within the magnetic field, the object will not be attracted to the magnet. When the object enters the magnetic field, the force of the magnet acts, and the object is attracted. The pattern of these lines of force tells us something about the characteristics of the forces caused by the magnet. The magnetic lines of force, or flux, leave the north pole and enter the south pole.