The result is that the zinc disc ends up with a lot of electrons "left behind" by the zinc ions, and the copper disc ends up "missing" a lot of electrons "stolen" by the zinc ions. To use the electricity that comes from the pile, we must connect something that uses electricity to the two discs at the top and bottom of the pile. Doing so gives the electrons left behind in the zinc disc a "chance" to go to the copper disc. At the copper disc, the electrons "replace" those that were "stolen" from the copper by the zinc.