What is Net magnetisation, rotating frame?

The net magnetisation $\mathbf M$ of a sample is the sum of its nuclear magnetic moments per unit volume; at equilibrium it is $M_0$ along $\mathbf B_0$. The rotating frame is a set of axes $x'$, $y'$, $z$ turning about $z$ at the frequency of the radio waves applied; in it the precession at $\nu_0$ is seen slowed to the offset $\nu_0 - \nu_{\mathrm{rf}}$.