DWR means Durable Water Repellent. It is a special coating added to fabric ta make it hydrophobic, water-resistant. If the DWR on the textile is fully functional, water droplets from rain gather on the surface of the DWR coating as round drops. The most common application is naturally for clothing, but also for the outer layers of tents or for sleeping bags. Thanks to the DWR coating, clothing or other treated surface is water-repellent and non-absorbent. DWR also helps prevent stains on the fabric.
For clothing, the DWR coating is usually used either on its own or in combination with a waterproof breathable fabric such as Gore-Tex. This prevents the outer layer of fabric from becoming saturated with water. This saturation, called 'wetting out,' can reduce the garment's breathability (moisture transport through the breathable membrane) and let water through. DWR acts as a repellent against rain and snow.
DWR resists to rain droplets, but even salt water does not disturb this coating. However, after some time, DWR can weaken and become less effective. Washing DWR coated fabrics using strong and inappropriate chemicals can definitely have negative effects and disturb it. Therefore, it is recommended to restore the protection using special impregnation sprays.