The substance which has lower reduction potential are stronger reducing agent while the substances which have higher reduction potential are a stronger oxidising agent.

$\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]^{3-}+\mathrm{e}^{-} \rightarrow\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]^{4} ; \mathrm{E}^{\circ}=0.35 \mathrm{V}$

$\mathrm{Fe}^{3+}+\mathrm{e}^{-} \rightarrow \mathrm{Fe}^{2+} ; \quad \mathrm{E}^{\circ}=0.77 \mathrm{V}$

The reduction potential of $\mathrm{F} \mathrm{e}^{3+/} \mathrm{Fe}^{2+}$ is higher, hence, $\mathrm{Fe}^{3+}$ is a strongest oxidising agent.