What is Polarity inversion?

The electronegativity of magnesium (1.31) is much lower than that of carbon (2.55): in $\ce{R-MgX}$ the C–Mg bond is polarised with the negative end on carbon, which behaves as a carbanion. In the halide $\ce{R-X}$ the same carbon was positive (X: 2.96 for Br). Turning an electrophilic carbon into a nucleophilic one is a polarity inversion.