Voltage, up close

Voltage is electric potential difference: the energy change per unit charge between two points, measured in volts. In an ordinary resistive DC circuit, a voltage difference drives current. The water-pressure analogy is useful within that simple model.

The crucial subtlety: voltage is always measured between two points — it is a difference, never an absolute. Saying "this point is 5V" is shorthand for "5V higher than our reference point (ground)". Just like altitude only means something relative to sea level, voltage only means something relative to a reference.

This is why a charged battery sitting on a shelf still reads its full voltage on a meter even though no current flows. The potential to push is there; it just needs a complete loop to act on.