An underexposure will produce solid shadows, with no visible details, while excessive exposure or overexposure will produce pale, uniform, pale surfaces. The calculation of the two factors, aperture and shutter speed, is performed automatically by the camera, leaving us free to concentrate on the scene you want to photograph.
However, it may happen that the image is too light or too dark even though the available light is sufficient for the exposure meter to function, ie the system that automatically calculates the exposure. This happens if, the scene that you framed can contain elements that mislead the sensor, distorting the result as shown by the famous Bob Willoughby.
Proper exposure is when the sensor of the camera has received an adequate amount of light. While it is said underexposed when the light is poor and overexposed when the light is excessive.