Humidity is the percentage of water available in the air. A high humidity level is extremely important during the growth of the mycelium and the fruiting period. Most types of mushrooms incubate best with a humidity level of more than 90 %. During the fruiting conditions it's wise to let the level of humidity in the air go down to 85-90 %.
You can keep up the level of humidity in the air if during the formation of primordia you spray it daily with water. During this period it's very important to moisten the air with a mist of water. Do not spray the water directly on the casing layer. Big drops of water are dangerous to the mycelium.
A better method to control the humidity in the air is to use perlite. Perlite is a great keeper of mist. Applying a layer of perlite on the terrarium with some water will keep the humidity on a constant reasonably high level.