What words do the letters in "LUFS" stand for?
Is your Google broken?
Loudness Units Full Scale. "Full scale" is the 0dB reference, and this is a
digital level and a
digital reference. Of course the actual acoustic loudness (dB SPL = sound pressure level) depends on the volume control and the size of your amplifier & speakers, etc .
LUFS takes into account the fact that low-frequency and high-frequency sounds aren't perceived as loud as mid-frequency sounds, and that short-duration sounds are not perceived as loud as long-duration sounds.
Similarly, dBFS is decibels full scale, which is the digital level relative to the "digital maximum".* That's why digital dB levels are negative. dBFS just looks at the digital amplitude without regard to how loud it's going to
sound.
With SPL, 0dB is approximately the quietest sound that can be heard so SPL levels are normally positive. Most SPL measurements are "A-weighted" which takes frequency into account, similar to LUFS. Although there is usually no
calibration between SPL and LUFS (or dBFS) there is a direct
correlation and if you reduce the digital level by 3dB, the SPL level will drop by 3dB, etc. (assuming everything is linear and the amplifier isn't being driven into distortion).
*0dBFS is the highest you can "count" with a given number of bits. The numbers in a 24-bit file are bigger than the numbers in an 8-bit file, but everything is automatically scaled to match the DAC when it's played so a 24-bit file isn't louder. With floating-point audio 0dB is equal to 1.0, so you can obviously go way over 0dB (but your your DAC is still limited to 0dB).