What does "breathable material" mean?

Moist skin cools down up to 20 times faster than dry skin, which is why kids can quickly feel cold after being active in the cold. That's why the best waterproof materials are also breathable: sweat can escape through the small pores in the fabric, rather than remaining on the skin. Material breathability is expressed as g/m2/24 hours, which tells you how much water can evaporate through one square meter of fabric in one day.