Iron is more reactive than $\mathrm{Cu}$. So it displaces $\mathrm{Cu}$ from copper sulphate solution. Thus, blue colour of the ${\mathrm{CuSO}}_4$ solution fades and colour of the solution turns green due to the formation of ferrous sulphate solution. This is a displacement reaction.
$\underset{\left(\mathrm{Blue}\right)}{{\mathrm{CuSO}}_4\left(\mathrm{aq}\right)}+\mathrm{Fe}\left(\mathrm{}\mathit{s}\right)\to \underset{\left(\mathrm{Green}\right)}{{\mathrm{FeSO}}_4\left(\mathrm{aq}\right)}+\mathrm{Cu}\left(\mathrm{}\mathit{s}\right)$