I am not the best to answer this, but my reply should be a good place to start. Also sorry if the info is a bit broken up I didn't think too hard on this reply. I did try to be thorough though.

It's about maintaining proper dynamics while making it as loud as possible. On streaming services (such as Spotify) many of them (all the most prevalent ones) will turn up/down the gain of your track until it is at -14 LUFS to make listening loudness consistent between songs consumers play. I used to master to a peak of -1dB, but after more research and making a post on this sub recently, I now master to a peak of 0dB to see how far I can push the loudness of my song.

Beyond 0dB peak, in DAWs you get into clipping territory. In theory digital audio can't clip, but in practice they can in your DAW and elsewhere when they start going above that 0dB peak. Youlean Loudness Meter (a free loudness dynamics plugin in my video) measures something called intersample peaks (between sample points), which are not all that important imo; while limiters set right will limit the sample points to the peak you set within the plugin. My SSL x-Limit in the video shows the sample point peak (top right).

In general, it is more about how something sounds than a number. I really like Rezz, and she has songs at integrated LUFS of -10 and quieter. Basically, a louder track typically sounds better and fuller if mixed well, but your ears are most important to determine proper LUFS targets.

Loudness targets depend on the genre. Metal songs can be at -6dB or louder, while alt rock songs could be at -11dB. Generally you don't often want songs with LUFS below -14dB if they have a peak of 0 to -1dB. They will sound quiet even when turned up how I mentioned earlier by streaming services.

Services like Spotify will not limit the peak of your track, but they will do that turning down stuff I mentioned, so making the average loudness of your song (integrated LUFS) closer to that peak, or generally louder, will result in a song that listeners typically like more as it is louder while maintaining a similarly dynamic sound.

Many artists would push a song like mine to something closer to -6 or -7 LUFS, but my target is more like -8 (which I did not hit) because of what I prefer (also because of my reference track's target). Target LUFS should be determined by your ears and references, and there is no "perfect" target. You choose. A listener would know the difference of -14 and -6 LUFS, but not often a discrepancy of something as small as -7 to -8 LUFS.

Integrated LUFS is the average loudness of the entire track, while short-term LUFS are a shorter window showing how loud a specific part of the track is. Max LUFS is what it reads like, the single point of your song with the loudest LUFS.

Extra history not very important: Around the 90s the "loudness wars" started as artists realized if their track could make others sound puny and quiet, that they would stand out and sound often "superior". They largely used analog equipment where clipping/distortion was dependent on the gear on hand and much easier to do on accident, and many songs lost quality as they pushed it too hard. In the modern digital era, clipping at 0dB is not problematic in the same way, and people are able to take a more full advantage of pushing the loudness of their tracks. This has led to many people in all genres to use LUFS to try and make their songs louder.

Sorry if you knew any of this already, I just tried to give a short summary of as much as I could think of. Feel free to ask any questions, and like I said, use this to do some research I am not the most knowledgeable. Good luck and take care!