If the settings of a camera are inadequately adjusted, especially in a dark area, the resulting picture is too dark and is called underexposed. It simply means that there is insufficient light to capture the image details completely. This image is generally a fail and is often rejected. There are three ways that you can fix this.
The first is by widening the aperture so that more light gets into the zoom lens. The other way is by managing the ISO and the shutter speed. These two adjustments will help you influence the length of time that the light is shining on the film, controlling the exposure of the picture.