Fruiting conditions means that you will place the open substrate with the casing layer somewhere where there's light, fresh air and lower temperatures.
When the substrate is totally colonized by the mycelium, you will add the casing layer. Then, put it back in the incubation room for a few days. When the mycelium comes through the casing layer, it's time to leave it open to the fruiting conditions. Thanks to placing the substrate in the fruiting conditions you give it the chance to form fruit bodies.