What is LUFS?
LUFS (sometimes LKFS) stands for Loudness Unit Full Scale, which is the new standard for measuring audio loudness, and it is the most accurate way to measure a track’s average loudness over time: it indicates how loud the listener perceives a track.
The online shops apply this in order to have a somewhat uniform volume across all tracks, despite the compression and mastering of the individual tracks. This is the reason why the listeners don't have to keep the volume knob at hand all the time!
The streaming shops will automatically normalize the track volume to their LUFS. Those numbers are constantly changing so check the FAQ of each shop to find out more.