Not all tents keep water out. Waterproof ratings show how much water a tent can block. Ratings start at 1000mm for light rain and go above 5000mm for heavy rain. Some materials, like silnylon with silicone, stop water better. Seams get sealed to stop leaks. Rainflies that cover the whole tent help too. These features make tents strong against rain. Choose a tent with these if you want to stay dry.