https://www.techradar.com/news/the-a-to-z-of-photography-underexposure

There are two sides to ‘underexposure’. There’s a technical side, where you can look at a photo’s histogram – a graphical representation of tones in the picture – and see if the darker tones have been cut off, or ‘clipped’. 

But there’s visual side too, where a picture just looks darker than you intended. That’s a creative choice that’s not always the same as getting a picture ‘technically’ right. Sometimes you want a picture to look dark and moody because it suits the subject, and sometimes you want an image that’s airy and light for the same reason.