Iron is more reactive than `Cu` . So Cu in `CuSO_4` will be displaced by `Fe^(2+)` in iron.
`Fe+CuSO_4 rarr FeSO_4+Cu`
Initially the solution will look like blue because of `CuSO_4` . But when `Cu` was replaced by `Fe` then `Cu` will be precipitate as a thin layer. We can see it by the change of colour from blue to red in the solution.
As explained in the early part `Pb` in `Pb(NO_3)_2` will be displaced by `Zn` .
`Zn+ Pb(NO_3)_2 rarr Zn(NO_3)_2+Pb`
We can see crystals of Pb forming on Zn.