There are two different types of materials that are commonly used with today's tents, they are; polyester and polycotton.
Polyester is a fabric woven/ knitted from a polyester thread and is normally backed by some form of PU (polyurethane) coating and treated with a DWR (Durable Water Repellent) coating, finally, the seams are then taped with waterproof tape. This material is incredibly waterproof and can hold many metres of water pressure on the surface of the fabric before you experience any type of water ingress. This fabric will stay waterproof for a long period of time, however, it is advisable that you reproof your tent every so often to maintain the materials water repellency.(Link Below)
Polycotton, on the other hand, is a blend of woven cotton and polyester thread - the details of this blend is usually in the product description of the tent - this fabric is also treated with a DWR coating.However, unlike polyester, it cannot receive an adhesive coating to the fabric without this the seams cannot be taped and the needle holes from where the seams where sewn may be visible and can let water in if the tent is not weathered beforehand.
Weathering basically means wetting the fabric, and this process lets the cotton in the fabric soak up any moisture and expand. Thus closing the needle holes and any other small holes in the fabric. Once this has been done, you should dry out the tent and then there should be no need to do this again.
You may, however, need to top up on some proofer after weathering your tent.
Q. Should I Weather my Polyester Tent?
A. No, it does not need to be weathered.
Q.How should I weather my tent?
A. Pitch your tent then wait for the rain, if this doesn't work a good 10-minute soak with a hose will be fine.
Q.Should I weather my tent every time I am away?
A. Nope, once should be enough to get the cotton to expand.