Magnetic flux

Magnetic flux is what generates the field around a magnetic material. It consists of photons, however, unlike the light we receive from the Sun, it is at a much lower frequency. (1) This is why magnetic field lines are not visible to the naked eye.
It is the alignment of "electrons" in the atomic shells of Ferromagnets and, separately, the “"spinning” electrons" in Electromagnets, that provides a material with it’s Magnetism. (2)
The number of magnetic field lines passing through a surface (such as a loop of wire). The magnetic flux through a closed surface (such as a ball) is always zero.
The SI unit of magnetic flux is the Weber (Wb) (in derived units: volt-seconds). 
The CGS unit is the Maxwell. 
Magnetic flux is sometimes used by electrical engineers designing systems with electromagnets or designing dynamos. Physicists designing particle accelerators also calculate magnetic flux.
References.
1 - https://www.rpi.edu/dept/phys/ScIT/InformationStorage/faraday/magnetism_a.html
2 -https://van.physics.illinois.edu/qa/listing.php?id=414