Question

Why is it that when something gains electrons, it is said to be reduced? What is being reduced?

Short answer

An unstable atom or electron is reduced to stable the state.Here, Cu² is reduced to Cu and now it has 0 oxidation number, due to gaining of electronsinCu²

Step-by-step solution

Example of Reduction and Oxidation

An example of reduction and oxidation reaction is, Zn(s) and Cu²

$Zn\left(s\right) + C{u}^2 \to Z{n}^{2+} + Cu\left(s\right)$

The above reaction isexplains how Cu² is reduced because it gained electrons.

Conclusion

Cu² Is reduced to Cu and nowit has 0 oxidation number, due to gaining of electronsinCu²