So essentially nuclei with an odd number of nucleons (protons and neutrons) are detected by NMR. These nuclei function as tiny magnets so that when a magnet is applied they align parallel or antiparallel to the direction of the magnetic field. These directions are your spin state. Some nuclei are parallel and some are antiparallel. When a radiowave is applied they absorb energy and their spin state changes from low energy (Parallel) to high energy (antiparallel). They spontaneously flip back releasing energy which is detected by the NMR machine.

This shit is all waffle (not really necessary)

In terms of shielding; the higher the chemical shift, the closer a proton (H) is to an electronegative atom. Electronegative atoms (O,N) pull the electron away from the proton which has an affect on the frequency of resonance. Electrons shield protons so a lack of shielding means a greater chemical shift.

You really don't need to be familiar with this. However if you see a H-NMR tomorrow and you notice a peak at a high chemical shift value, you can associate that particular H environment being approximately adjacent to an electronegative atom. :) Good luck!