Hydrostatic Head (HH) is a way of measuring how waterproof a fabric is. This is shown in millimetres as 10,000mm HH and it shows how much force would be needed to push water through the fabric, getting it to leak or stopping it being waterproof. 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 by using a actual column of water standing on the fabric and seeing how high that column would need to be before the water would penetrate the fabric. However, as fabrics have developed the ratings were getting too high for this method. Having somewhere that could hold a 30 metre high water column is a bit unrealistic even for test centres. Instead, modern tests use a machine that replicates the 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 amount of pressure needed is converted into millimeters and this provides the result of the HH test.
Hydrostatic Head in Tents: In order for a tent to resist light showers the HH needs to be around 1000mm. For heavy rain and driving wind (typical in the UK) this 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 or pack leaning on it.
A groundsheet needs to have a higher HH figure because of the pressure of people pressing down on it when lying or kneeling inside their tent, so it should ideally be 3000mm or higher.
Hydrostatic Head in Clothing: In the UK manufacturers are allowed to claim a fabric is waterproof if the HH is 1500mm and over. In reality most waterproof jackets go well beyond this and figures of 10,000 to 30,000mm are pretty common. Fabric used for clothing needs higher levels of waterproofing because they have to deal not only with driving rain but also the pressure applied by straps and hipbelts of rucksacks. It is important to remember that a higher HH does not guarantee a better piece of gear since breathability is usually compromised when water proof performance is maximised. This means that although the jacket may stop water getting in it may also struggle to let it out - leaving you hotter, sweatier and in the end, wetter.
Softshell jackets and pants come in a range of different types from lightweight, highly breathable pieces that won't put up with much rain, to heavier more water resistant garments that are often made from waterproof fabrics - but the garment is not called 'waterproof' because the seams aren't 'taped' (sealed shut).