What is Numeraire, money-market account?

A numeraire $\mathcal N$ is the price process of a traded asset (or self-financing portfolio) that is strictly positive at all times; values expressed in units of it are $V_t/\mathcal N_t$. The money-market account is the numeraire $B_t = \exp\int_0^tr_s\,ds$ that rolls overnight at the short rate $r_t$.