Voltage is a separation of charge.
As seen in the figure below there is a lack of electrons in the air and an excess of electrons on the dome.We will call a single electron one electron volt. Notice there are + signs in the air molecules where electrons are missing, if we add all these up we see that the air is missing 6 electron volts. If we look in the red area on the dome we see that there is an excess of 15 electron volts on the dome.
If we just count the two areas circled there is a voltage difference of
-15 - 6 =-21 electron volts.
This seperation of electrons or charge is called voltage.