Hydrostatic Head (HH) is a measure of the level of waterproofing present in a tent. It relates to the pressure of water that the tent material can withstand without leaking. In real terms this means how hard the water is hitting the tent material. i.e. how heavy the rain is.

A very rough guide:-
HH1000 Waterproof in showers and normal rain. Fine for a usual British Summer!
HH2000 Waterproof in heavy rain, but may start to leak if heavy rain is prolonged for too long.
HH3000 Can take heavy rain, the odd torrential downpour and most of what nature is likely to throw at it. Most people would get fed up and go home before these tents leak!