Final answer:
Moving the middle Input slider to the left in a Levels adjustment layer increases the midtones, making the image appear brighter and enhancing midtone details.
Step-by-step explanation:
When applying a Levels adjustment layer to an RGB image in photo editing software like Adobe Photoshop, and you move the middle Input slider (also known as the gamma slider) to the left, you are effectively lightening midtones. This action increases the input range for the lighter areas, thus brightening the midtone values without significantly altering the highlights and shadows. As a result, the overall image appears brighter with more pronounced midtone details, which can help in correcting images that are slightly underexposed or lack contrast.