When it comes to synthetic tents, you might have seen the term HH floating around, and this stands for hydrostatic head. This is one of the characteristics that tents will feature to ensure that they are water resistant.
The number next to HH will represent the height of a water column that the fabric will be able to resist. So, if you are camping in the rain the more water that is raining onto your tent, the higher the pressure of the water will be applied to the surface. At the point where the pressure is maximized, the height of the column can be measured, and this height in millimeters is the hydrostatic head of the fabric.
So, if the tent has a HH of 4000, it will be able to hold a water column that is 5000 mm tall. You will also need to consider other variables, like the seams and zippers of the tent, but the higher the HH value is, the more water resistant the tent will be. Though, the overall structure of the tent can also play a part in how efficient it is at blocking water.