Resin is a type of propylene plastic used extensively in outdoor applications.
In other words; resin is a synthetic material. It is a kind of plastic. Most of the white or green chairs you see stacked up are made out of resin. Chaise lounges in many beach resorts are made out of resin.
Resin is used in outdoor applications because it is durable. It does not rust, mildew or fade. Of course depending of the quality of the resin it might be different. Some bad quality resin products might start fading under sun after being used for long periods.
Virgin resin products are better in quality then recycled resin products. Recycled resin products are blended with bleach to make white color. This harms the characteristics of virgin resin. Resin products should be made from virgin resin.