Hydrostatic Head is a term used to describe the waterproofness of a particular fabric. For any fabric to be considered fully waterproof it must be able to withstand the pressure of a column of water 1000mm high without leaking. This is quantified as a hydrostatic head rating of 1000. The Ministry of Defence definition of a waterproof fabric is that it must resist a column of water at least eight hundred millimetres high - that is to say a hydrostatic head rating of 800. Most tents have a minimum hydrostatic head rating of 1500 and European weather, it is widely thought, will be repelled by a tent with a hydrostatic head of around 2000mm.
The optimum position for a shelter is that the groundsheet has at least the same, it not higher, a rating than the fly as the serious pressure will come from the knees/feet/backside pushing the groundsheet against the wet ground rather than rainfall. Indeed, the higher the rating, the longer it retains it waterproofness over the short term (during a specific trip) as well as the longer it retains it waterproofness over the long term (during its lifetime).
Hydrostatic Head is not the only issue in shelter design of course - shape plays a part too and this will have an impact on how much of the shelter is actually exposed to wetness. Condensation will also play a part - so ventilation is also key.
My Big Agnes Fly Creek UL1 has a HH rating of 1200mm on all the fabrics used - fly and groundsheet specifically - which means it is technically "waterproof" but is right at the bottom of the range - it is a 3-season shelter at best. Conversely, the Vaude Power Lizard UL has a HH rating of 10,000mm on the groundsheet which means it will likely be waterproof longer in both senses of that analysis.
The Hilleberg Akto - considered to be one of the better All-Season solo tents in existence - has a fly with an HH rating of 2000mm and a groundsheet with an HH rating of 5000mm. This is a true All-Season tent in terms of its ratings (but remember: waterproofness is not the only issue).
The TN Laser Competition has a fly with an HH rating of 3000mm and a groundsheet with an HH rating of 5000mm. It is perhaps the perfect balance of lightweight and waterproofness.
What does all this mean? At the very least, you'll have an idea how waterproof your fabric is. Thus, when pressing a knee into a groundsheet on very wet ground, you might, after a lengthy period of use, experience water ingress on a lower HH fabric. After some years of use, this may also be the case. Some users have reported water ingress on the Big Agnes Seedhouse SL2 after weeks of trekking and serious rainfall. That may be so, but there is always a compromise to be made between fabric and weight. Durability and waterproofness are always going to be issues in UL fabrics until technology advances. Being fully informed is one thing but being paranoid about kit is another.
In the end, backpacking, hillwalking and mountaineering is all about compromises and sacrifices. Know your limits and your destination and pack accordingly. Opinions will differ, of course, especially in this area as we can all survive with differing levels of comfort and we're seeking different theatres in which to spend our time.