What does this equation mean?

Plotted against the two real components of $\phi$ , this is the shape popularly called the Mexican hat: a local maximum at $\phi$ = 0 , surrounded by a circular trough of degenerate minima at |$\phi$| = $\mu$/$\sqrt{2\lambda}$ . The sign of the mass term is the entire mechanism. With $\mu^2$ < 0 the origin would be the unique minimum and there would be nothing to discuss; it is the negative mass-squared term, an assumption built into the Standard Model’s Higgs sector rather than derived from a deeper principle, that manufactures the trough in the first place. Nothing in the Lagrangian singles out any point on that trough. A system placed at the unstable summit will roll downhill in a direction fixed only by whatever infinitesimal fluctuation happens to nudge it first, and the direction it lands on becomes, from that moment, the vacuum every subsequent measurement will be made against.