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The light_environment entity is a light controller, that casts directional lighting from the skybox. This is to approximate the light cast from the Sun. The first keyvalue, Brightness, controls direct lighting. This approximates direct sunlight. The second keyvalue, Ambient, controls indirect lighting from the sky. This is the diffused light that is cast on every face that can "see" the sky.
If you don't want to come up with your own values for this entity, the values used in the official Valve games for each skybox, are given here.
Normally, only one light_environment entity is required per VMF file. A 3D Skybox requires placement of an extra light_environment. If multiple light_environment entities are placed, Vrad uses the keyvalues from the first placed light_environment to calculate the lighting.