The lowest geographic place in the United States is Badwater Basin in Death Valley. It’s altitude is -282 feet. That’s a negative altitude.

What does that mean? It means that Badwater Basin is below the 0 altitude level, which we have chosen to define as “mean sea level.” We could have chosen a different altitude too be zero, but we didn’t.

If there is a pathway from the sea (at Mean Sea Level) to Badwater Basin, then the water will flow to the basin, because it would have a lower potential energy in the basin.

Voltage is potential difference. It is always the difference in potential between two different points. But like altitude, we often don’t say one of the two points out loud because it’s a common reference level.

A positive charge in a vacuum will gain speed (kinetic energy) moving from a higher potential to a lower potential, because its potential energy goes down. It doesn’t matter whether it’s going from +9V to +5V versus 0V to -4V. The change in potential is the same.