The property that allows some nuclei to behave like magnets is called Spin. Hydrogen nuclei are used in MRIs, because Hydrogen is present in all tissues. A Hydrogen atom is a Proton. These Protons have small N and S poles, and they are in random directions under normal circumstances (gravitational field).
When a strong magnetic field is used, the protons are forced to align itself with the field with the N facing the S pole of the field. Occasionally, a proton is in a flipped position compared to the rest (S pole to S pole) - this is an unstable higher energy state.
The protons rotate in the direction of the magnetic field rather than align exactly with it. The rotation action is called Precession.
The angular frequency of the precession is known as the Larmor Frequency, which depends on the nucleus' involved and the magnetic flux density of the field (the `frequency` is measured using Radians per second). The stronger the external magnetic field is, the faster the protons precess.
The protons in the external magnetic field are occasionally subject to pulses of (radio frequency) waves. This frequency equals the frequency of the precession, so resonance occurs (nuclear magnetic resonance).
When the RF waves are switched off, the protons gradually revert back to the lower energy states - by releasing RF waves. We can detect these, and the rate of the `relaxation` of the protons can be used to tell us about the environment of the protons. For example, water/watery tissues have a relaxation period of several seconds, fatty tissues last several hundred milliseconds, but cancerous tissues has an intermediate period.
This allows an image of the patient to be built up.
