What is Short-rate model?

A short-rate model specifies the dynamics of the instantaneous rate $r_t$ under the risk-neutral measure $\mathbb Q$ (One Quant Book 4, chapter 5), and prices every zero-coupon bond as $P(t,T) = \E^{\mathbb Q}_t\bigl[\exp(-\int_t^Tr_s\,ds)\bigr]$.