A hydrophobic coating is a substance placed on the glasses that can help keep wick away moisture or repel water. This is is especially important in a pair of running sunglasses since most of us sweat quite a bit. The last thing you want to do at the end of the day is wearing a pair of sunglasses that slip around and fall all over the place while you are on the go. The hydrophobic coating is an incredibly important factor to consider if you want a pair of glasses that you can rely on and won't drive you crazy while you are out running.