Your video's loudness for social media
You upload the video and it sounds quieter than everybody else's, or it crackles. Here you measure the real loudness, the one platforms use to even out every video (LUFS), and you are told how many dB you are away from YouTube, TikTok, Instagram or Spotify. The file stays on your device.
Why your video sounds quieter than the others
Because platforms do not leave it as you made it: they even it out. Every video is measured and brought to roughly the same loudness as the others, so viewers do not have to touch the volume at every scroll. If yours is louder than the reference it gets turned down, and all the effort spent making it «sound loud» is thrown away; if it is quieter, almost nobody turns it up, and it stays weaker than the rest forever. There is only one thing you need to know: how far off you are, and in which direction.
LUFS are not the peak (and that is the whole mistake)
The meter you see in every editing program shows the peak, that is the highest instant: but the loudness you hear depends on how high the sound sits on average, and on what it is made of. LUFS are that measure, and the standard that defines them (ITU-R BS.1770) does two things a plain average does not: it runs the sound through a filter that imitates the sensitivity of the ear (which hears mids and highs better than deep bass), and it throws the silences away with two gates, otherwise the pauses between one sentence and the next drag the number down and normal speech looks whispered.
The true peak, the one that distorts after you upload
The samples of an audio file are photographs taken forty-eight thousand times a second: between one and the next the waveform carries on, and it can go higher than the last sample. Your program says 0 dB and everything looks fine, then the platform re-encodes the file, the real curve is rebuilt, and that is where it distorts. Here the peak is measured for what it will really be (dBTP), not for how it looks in the samples, and if it goes over -1 dBTP you are told. It is why people who work properly always keep a finger of air above.
What to do with the number
If you are below the reference, turn everything up by the difference shown: that operation does not change the sound, it just moves it. But watch the peak: if turning it up would take you over zero, turning it up means distorting, and what you need is a limiter at the end (or a mix redone a little lower). If instead you are above, know that you will be turned down anyway: squashing the sound to «be louder» achieves nothing except making it sound flatter. The dynamics figure (LRA) next to it says exactly that: under 3 LU the piece is squashed, over 10 it breathes a lot.
What it does not do
It does not touch the file: it only measures, and when you close the page nothing is left. It does not change the loudness, does not normalise and does not apply limiters: to actually change the volume of an audio there is Change speed and volume, and to clean up a badly recorded voice there is Improve audio and voice. It reads the formats your browser can decode (MP4, MOV, MP3, M4A, WAV, WebM): if the file is in a rarer format it says so, and then it is worth going through Convert video to MP4.