What does the 'mm' waterproof rating on a tarp actually mean?

It's the hydrostatic head, the height of a water column the fabric can hold before leaking. 1,500mm is the technical threshold for 'waterproof,' 3,000mm handles most camping rain, and 20,000mm ratings (seen on heavy bushcraft tarps) are overkill for casual use but reassuring in sustained storms. Seam taping matters as much as the rating itself.