Queue file used by Indigo Renderer, an application used to generate photorealistic images; contains the scene and output paths, along with other queue settings.
The scene path points to the .IGS file location from which the scenes are being rendered, whereas the output path points to the location that the files (most likely .PNG) will be created. The output path can be relative to the scene path or absolute.
The IGQ file allows you to render scenes from projects created in other applications, such as 3DS Max or iClone, where the Indigo Renderer plugin is installed. You can export the project frame by frame into the IGQ file and Indigo will individually render each frame. Then, you can combine the rendered images into an animation.