Hysteresis loop is basically a graph in magnetism. To understand how it forms and to make things clear, first we need basic understanding of some magnetic terms

1. Magnetic Field Strength: It is magnetic field quantity. Using this we can express the intensity of a magnetic field of a magnet. It an be visualised by magnetic field lines that form around the magnet. The strength is simply the density of these lines. It is denoted with variable H, and measured in Amperes per meter (A/m)  [1]
2. Magnetic Flux: The total number of magnetic field lines passing through a surface (such as a loop of wire) is called magnetic flux. The magnetic flux through a closed surface, such as a ball, is always zero. The density of this, called the magnetic flux density is denoted by B, and measured in Newton-meters per ampere, also called as teslas, T.  [2]
3. De- magnetisation : To store magnetic power in materials, we impart magnetisation and polarity on materials in various applications. This process is called magnetisation. The reverse of this is demagnetisation.
4. Ferro magnetism: It is a basic mechanism by which certain materials form as permanent magnets or attracted to magnets. [3]

Now, consider a completely demagnetised ferro magnetic material (for which, B=H=0). If we subject this material to increasing values of H and measure the corresponding values of B, it will form a graph as shown below. [4]

After a point ( b in this case), if we try to increase the value of H, B won't get increased anymore. This point is called saturation point. If we decrease the value H, the curve follows b-c path. Which means, the material will have some value of B even the value of H is zero. This is called remanent flux density.

If we further  decrease H, it will decrease upto point d. Here B becomes 0.

If we further decrease H down, the curve follows the path d-e, increasing H in reverse direction and stops at the saturation point, e.

If H varied backwards, then the curve follows e-f-g-b path as shown above.

The loop formed is called as hysteresis loop.  Using this loop, we can determine various properties of a material such as retentivity, permeability,  and reluctance [5]

[1] https://whatis.techtarget.com/definition/magnetic-field-strength

[2] https://simple.wikipedia.org/wiki/Magnetic_flux

[3] https://en.wikipedia.org/wiki/Ferromagnetism#Ferromagnetic_materials

[4] http://www.electricalbasicprojects.com/what-is-hysteresis-loss-hysteresis-loop/

[5]

https://www.nde-ed.org/EducationResources/CommunityCollege/MagParticle/Physics/HysteresisLoop.php