Voltage is produced by a chemical reaction. Inside a battery there are positive and negative lead plates that sit in a liquid called electrolyte solution. Electrolyte solution is a mixture of water and sulfuric acid.

When this solution interacts with the lead plates, there's a chemical reaction. This chemical reaction creates pressure that we call "voltage."

The pressure (voltage) pushes the current out of the battery's negative terminal through the "load", the equipment drawing the power. The current returns through the positive terminal.