We can illustrate the direction and strength of a magnetic field by means of drawing 'flux lines'. These have arrows to show the direction of the magnetic field at any point and the higher the field, the closer together the flux lines.
So the flux density (i.e. the number of flux lines per m2) at a given position is proportional to the strength of the magnetic field at that point.
The term µ µ0 H is defined as the flux density
N.B. Flux lines are always continuous. So when we draw diagrams of flux paths, they may only be part of the actual picture