Batteries are devices that, from a redox reaction, produce an electric current. Basically, the battery turns chemical energy into electrical energy. The oxidation reaction takes place on the negative electrode, and electrons flow out of the cell.
Oxidation is the loss of electrons and an increase of electrons is a reduction. That is, oxidation is often connected to a decrease. If something gets oxidised, the electrons are obtained by another agent, acting as the oxidising agent, and in the process is reduced.