You got a load of MP3s from different sources, different CDs, downloads etc, maybe even vinyl, and at different rates, and when you're playing them together, say in shuffle, some are louder than others...it's annoying.
Three's something called normalizing, and the idea is that it matches the various MP3's differing volume levels together, to an overall constant volume level. I assume "Smart Volume" is a Samsung feature, and it sounds like it's not working as it should. Apple calls it "Sound Check", that iTunes, iPods and iOS have had for a long time, and I found it does work quite well.
My Oppo has something called MaxxVolume, which basically does the same thing, and that works OK as well for ensuring a consistent listening level, no matter the sources.