On the first step, electrons from the C-Br bond move to Br, the C-Br bond breaks, and Br^- leaves with the negative charge.This step is rate-limiting and produces a carbocation.

On the second step, the carbocation pulls an electron pair from Nu, forming a C-Nu bond.

On the second step, the nucleophile can add from either side of the carbocation with nearly same probability, so the stereochemical configuration of this carbon is lost.