Relative humidity, written RH, is how much water the air holds compared with the most it could hold at that temperature. A mushroom is mostly water and loses it fast to dry air, so caps crack and edges curl.
DMR gives 75 to 85 percent RH for oyster fruiting, 80 to 85 for button cropping and 80 to 90 for milky. But wet is also a disease. A film of water lying on caps at 20 degrees or more lets bacterial blotch spread, and NHB says to avoid more than 85 percent RH together with more than 20 degrees.