If you dig into this topic at all, you will hear about the "red book CD standards". These can be pretty confusing and from what I understand, they are not used much except among the "audio snobs" and mastering houses as a scare tactic to get you to use them.
Essentially, what you need to do is learn how to get the best level out of your project in the end. And in order to do that, you want the tracks at the start to be nice and full, strong tracks and not skinny wimpy things. AS you progress through the various steps, you want to be sure you are keeping all your levels at decent levels. Not into the clipping and not way down either.
Loudness wars were mentioned. That's a thing of the past for the most part.
I like to use a DAW and an editor for my mp3/waves that lets me see the waveform I have. That waveform can tell you a good bit about the file you have.
This first photo is a wave that has way too much compression on it. I guarantee you, it's loud. But there are also no dynamics. This is an example of the loudness wars and what files looked like when they were party to the wars. You don't really want your files looking like this. I did this simply as an example..... turned the compressor settings up to the max.....or close to it. Notice there is no headroom above the wave and very little drop from 0dB at all.
Example #2 is on my music page.... In a World without you. This one is quite different from the first. Lots of head room and lots of dynamics. This song is relatively loud but not overly so. You will also notice that not a single spike hits 0dB. everything is well below the point of clipping and in spite of that, there's a good sound level that doesn't have anyone reaching for the volume control to turn this up.
Most songs that I hear in the showcase and on other sites tend to be low in comparison. The way to check to see if your music is low, high or just right is easy. make up a play list of songs.... mix in your originals and commercial stuff off of a CD from a big name artist. If yours are any different in volume, start working on getting them up to that level.
It's not magic but it's also not simply dialing the gain, normalize, limiter, compressor or whatever else you use up to the max. The trick is to get the volume and keep the dynamics.
Last example. This is a file more compressed but still with some remaining dynamics and headroom above it. This file could be bumped up a few more dB of volume before burning it.
You don't want to over do it to the point where you can hear the compression working or evident in the final song. On the other hand, if you notice that most of the other music from home studios is louder than yours, you probably could stand to turn it up before you do that final export to call it done.
