Another metal-ion equilibrium can be connected such that it provides a way for electrons to flow.
For an unreactive metal, the equilibrium lies mostly to the left:
E.g. Cu(s) Ý Cu2+(aq) + 2e-(aq)
So this equilibrium “takes” electrons from the equilibrium of the more-reactive metal.
Each is a half-cell.
Two half-cells make a cell.