Olefin is made from 2 soft plastics known as polypropylene and polyethylene. Olefin is waterproof because it is weatherproof, strong and durable. Olefin material is used in a great number of products because of its ability to be used outside for patio furniture, toys and other things that may be left out in the elements.
1 Additional Answer
No, olefin is not waterproof. It does absorb very little moisture, and will wick it through very quickly, meaning it will dry quickly, but it will not prevent water from absorbing into the fabric. You can find more information here: en.wikipedia.org
Waterproof or water-resistant describes objects relatively unaffected by water or resisting the ingress of water under specified conditions. Such items may be used in wet environments or under water...