Chemical effects of electricity depend on ions – electrically charged atoms or groups of atoms. If an atom or a group of atoms gain electrons they become negatively-charged. If they lose electrons they become positively charged.
When two different metals are immersed in an electrolyte, one loses electrons and becomes positive. The other gains them and becomes negative. Negative ions in solution are attracted to the positive plate, and positive ions to the negative plate, and a chemical reaction can occur.
In a lead acid battery, the electrical and chemical differences between the sets of plates creates a potential difference which makes the current flow in a circuit. It flows in one direction only so it is called direct current, or DC. As electrons move from one set of plates to the other, the same compounds form on the plates.
If this happens for too long there will be no difference between the plates and current stops, leaving a discharged battery.
Of course, a battery can be recharged so that the difference between its sets of plates is restored.