ما معنى Pillar, instantaneous forward rate؟

A pillar of a curve is a date at which the curve carries a free parameter, here the continuously compounded zero rate $z(T)$, so that $P(T)=e^{-z(T)T}$; the curve between pillars is fixed by an interpolation rule. The instantaneous forward rate seen at $t$ for time $T$ is

the rate at which one can lock in, at $t$, borrowing over $[T, T+dT]$.