Hydrogen nuclei (protons) have magnetic properties, called nuclear spin. They behave like tiny rotating magnets, represented by vectors.
The sum of all the tiny magnetic fields of each spin is called net magnetization or macroscopic magnetization. Normally, the direction of these vectors is randomly distributed. Thus, the sum of all the spins gives a null net magnetization.
Within a large external magnetic field (called B0), nuclear spins align with the external field. Some of the spins align with the field (parallel) and some align against the field (anti-parallel).
Main nuclei imaged in human MRI