This is similar to measuring height. We measure the height of hills the same way. We say the sea level is at zero feet and then compare other points to that level. In the same way we call the lowest voltage in a circuit zero volts and give it the name ground. Then all other points in the circuit are compared to that ground point.
Rivers always flow towards sea level and currents always flow towards ground.
A battery is similar to a dam. On one side is a lot of stored up energy. When a path is formed from that side to the other side then current flows. If there is no path then current does not flow and the energy just stays there waiting for a path to form to the other side. The path can be a big path with lots of current flowing or a small path with just a little bit of current flowing.