The waterproofing of a tent indicates how much water its outer fabric can repel. It is measured in millimeters (mm) and refers to the number of millimeters of a vertical water column (and thus pressure) that one square centimeter of fabric can withstand without being penetrated by water.
The higher the degree of waterproofing, the less breathable the fabric is, and thus the warmer the tent. Common waterproofing values range from 500mm to 10000mm. Theoretically, any fabric with waterproofing >1000mm is considered waterproof. Indicatively, a summer tent may have waterproofing from 500 to 1500mm.