Outdoor fabrics need to be a number of things to be able to do their job properly. Polyester fabric is Water-Resistant, durable and UV-resistant, and it can be washed.
Polyester is a synthetic fabric that's usually derived from petroleum. Many patterns, colors, and styles are easily available in polyester. Polyester fabric is strong, quick drying, and often inexpensive, making them perfect for outdoor upholstery, cushions and pillows. For those on a budget, it is almost certainly the best choice for outdoor fabric.
Water resistance is an important factor in outdoor fabric, but really, the goal should be to avoid getting the fabric exposed to water. Even though the water won’t harm the fabric, you’ll still have to wait for it dry out in order to be usable again.