The Input Levels slider is responsible for most of the work when improving your photos, however the Output Levels slider can also play a part. Moving the white slider to the left reduces the value of white pixels so that in reality, the final image will include no white pixels. Moving the black slider to the right will make black pixels become dark gray.
This effectively reduces the contrast of the image, but this can be used to counter blown out highlight areas in a photo, for example small areas of bright white clouds in a darker sky. Dragging the white slider left will make pure whites pale gray and this can help to recover small areas of a photo, though if the blown areas are too large, the image will start to look flat.