U for Under-exposure
The photographic condition, when enough light is not received by the photo, we term it as an under-exposure. The overall photo appears to be darker and needs corrective tweaking in post processing. Most likely causes being high shutter speed and/or small aperture values. In some cameras, the RAW files capture more details in the shadows than in the highlights, hence having an under exposed photo is more usable than an over exposed one. When in doubt, under expose the scene by one stop, that’s my suggestion.