The waterproofing of a tent indicates how much water its outer fabric can repel. It is measured in millimeters (mm) and refers to how many millimeters of a vertical water column (and thus pressure) a square centimeter of fabric can withstand without being penetrated by water.
The higher the waterproofing rating, the less breathable the fabric is, and thus the warmer the tent. Common waterproofing values range from 500mm to 10,000mm. Theoretically, any fabric with waterproofing >1000mm is considered waterproof. For example, a summer tent may have waterproofing from 500 to 1500mm.