Your car’s battery carries a charge measured in volts. The battery produces this charge through the chemical reaction produced when a positively charged lead pole and a negatively charged lead pole are immersed in a sulfuric acid solution. The charge builds to a given potential regulated by the amount of oppositely charged poles used. The charge in a battery is measured using a voltmeter.
A voltmeter is a device that measures the electrical potential difference between tow oppositely charged units. In the case of your car’s battery, these units are the positive and negative poles.