What does the waterproof rating (mm) on a tarp mean?

The mm rating is hydrostatic head, a measure of how much water pressure the fabric resists before leaking. Roughly 3,000–5,000 mm handles sustained rain for backpacking weights, while 10,000–20,000 mm ratings (as advertised on heavy-duty tarps like the AquaQuest Defender) target extended storms, bushcraft, and ground use. Seam taping matters as much as the headline number.