The phrase "to dry the mushrooms" means to remove the moisture from mushrooms by exposing them to heat or air, so that they become dry and can be stored for a longer time. Full definition
I know that last part might sound complicated but all you have to do is soak dried mushrooms in hot water until they release their flavorful juices. (makingthymeforhealth.com)
To make mushroom flavor more pronounced, I also added one ounce of dried mushroom stir - fry mix. (chefdehome.com)
I make chicken stock and use dried mushroom stock or plain water as a base for many dishes, but never a vegetable stock. (mixedgreensblog.com)