The waterproofing of a tent shows us how much water its outer fabric can repel. It is measured in millimeters (mm) and refers to how many millimeters of vertical water column (and therefore pressure) a square centimeter of fabric can withstand without being penetrated by water.
The higher the waterproof rating, the less breathable the fabric is and therefore the warmer the tent. Common waterproof ratings range from 500mm to 10000mm. In theory, any fabric with a waterproof rating >1000mm is considered waterproof. For example, a summer tent may have a waterproof rating from 500 to 1500mm.