When nuclei with spin are placed in an external magnetic field, they either line up parallel to the magnetic field, or antiparallel to the field. The parallel state is much more energetically stable than the antiparallel state, but if you give the nucleus enough energy, it can flip to the antiparallel state. This is known as magnetic resonance. Hydrogen resonance is therefore the name of the process where hydrogen nuclei flip from their parallel to their antiparallel state.