Hydrostatic head or sometimes just HH is an outdoor industry standard way of measuring how much force it takes to force water through a piece of fabric.
The way that it is measured is by clamping a piece of fabric over the bottom of the tube and slowly filling the tube with water.
The hydrostatic head measurement just refers to how high the water got up to tube before it forced its way through the material.
An example would be a tent for summer use with a hydrostatic head measurement of 1000 mm. This means that the water in the tube got to 1000 mm high before it pushed its way through the fabric.
However that doesn't mean that your tent will be able to go 1 m underwater before leaking because the hydrostatic head test is in a controlled environment where there is very little movement of the water and the pressure is increased quite gradually.
A hydrostatic head measurement of just 1000 mm really only equates to a shower of rain.
Where you have a material which is used in a jacket or something which is going to be rubbing and pushing water through it the hydrostatic head measurement will need to be in excess of 5000 mm in order to be considered properly waterproof.