LUFS (Loudness Units before Full Scale) is a synonym for Loudness, K-weighted, relative to Full Scale (LKFS), which is a loudness standard that aims to normalize loudness levels across broadcast mediums. In other words, it’s not a scientific standard in line with ISO 80000-8 and other electrical engineering standards, it’s specifically measuring perceived loudness. The lower the number the quieter the level: -1 LUFS is louder than -10 LUFS, and -13 LUFS is quieter still.
This means that YouTube has decided that while the “loudness wars” — with everyone trying to master their music at the loudest possible perceived level — may be going on in the audio world, in the video world, they’re going to ensure that the audio in their streams has a controlled maximum level. It also means that with YouTube now being one of the most popular methods of music discovery and listening, significant incentive for the loudness wars may have effectively been removed.