Voltage is the difference in potential electrical energy between two points. In a 9V battery, the nine volts represent how much potential juice you’ll get out of it at a given time in the form of current.
Electrons flow from points of high voltage to low voltage, usually through wires and components. You can imagine voltage levels in a circuit as being like a contoured landscape. A ball rolls from a tall height to a low height, as long as there is a continuous path for the ball. Voltage is how much potential energy there is at the top.
Here’s a nice introduction to electrical voltage by SparkFun (source link here).