Waterproofing is a process where a particular objects or materials are treated with differently, or they are made out of materials or structure which makes it water resistant or water repelling. Materials can be different and in general, they are covered with a protective layer to prevent the water from going through.
There is no 100% waterproof material. Water will go through any surface at a time and with enough pressure. As all of the materials and surfaces will eventually get water inside, there is a set standard which can tell how good the material is and how water resistant is.
Waterproof and water repelling materials are not the same, and they have different qualities and features. If the material is water repellent, it means the liquid will not stay on the surface, but in case it stays on it penetrate much easier. Some materials like nylon are more likely to be waterproof, and some like cotton, have a low waterproof and water resistance ability.
For a better understanding how good the material is, no matter of purpose, there are standard set numbers which can help determine how good waterproof or water repellent material is.