Hydrostatic Head (HH) is a way of measuring how waterproof a fabric is. The resulting measurement in millimetres relates to how high a column of water standing on the fabric would need to be before the water would penetrate the fabric. For the best fabrics results can be as high as 30,000mm, i.e. 30 metres high, before it would penetrate the fabric.
Traditionally the test is carried out using a physical column of water but as fabrics have developed the ratings were getting too high for this method. Having the facilities to hold a 30 metre high water column is a bit unrealistic even for scientific test centres, therefore the test is done using a machine that replicates the downward pressure that a water column of such height would create. The machine increases the pressure of water that is pushed against the fabric until water is visible on the other side. The required level of pressure used to force water through the fabric is then converted into a measurement of how high the water column would have been. This provides the result of the Hydrostatic Head test in millimetres.
In order for a tent to resist light showers the HH needs to be around 1000mm, heavy rain and driving wind will create more pressure on the fabric and require a higher HH - around 2000mm. Anything above these figures and the tent will keep out water being pushed through by something physical, like a person leaning on it.
A groundsheet needs to have a higher HH figure because of the pressure of people pressing down on it, and should be around 3000mm or higher ideally.
In the UK manufacturers are allowed to claim a fabric is waterproof if the HH is 1500mm but most waterproof jackets exceed this comfortably and figures of 10,000 to 30,000mm are not uncommon. Fabric for use in garments require higher levels of waterproofing because garments are subjected not only to driving rain but also to pressure applied by straps and hipbelts of rucksacks. Gore-Tex and eVent fabrics typically achieve figures of HH up to 30,000mm making them the most waterproof garments around, however some fabric manufacturers, most notably Polartec, argue that these figures are not necessary, and by accepting a figure of 20,000mm or thereabouts you can make a fabric considerably more breathable - they introduced 'Neo Shell' which is becoming more popular and is a proven technology. A higher HH does not guarantee a better garment since breathability is usually a compromise when water proof performance is maximised. Softshell garments come in a range of different types from lightweight highly breathable that won't put up with much rain, to heavier more water resistant garments that are often made from waterproof fabrics - the garment is not called 'waterproof' because the seams aren't 'taped'.