When generating hydropower, the head is the distance that a given water source has to fall before the point where power is generated. Ultimately the force responsible for hydropower is gravity, so a hydroelectricity plant with a tall/high head can produce more power than a similar plant with a short/low head. In short, for a given water flow, a la...
see hydrostatic pressure 
[n] - the pressure at a given point in a liquid measured in terms of the vertical height of a column of the liquid needed to produce the same pressure
Measured vertical column of water determines how waterproof a fabric is.