Simply, if you notice on different recordings, the volume knob for one recording, you barely need to turn it on and the music is very loud. Other recordings you can turn the volume knob half way before it gets loud. The one that you barely have to turn the volume on is what we would call very compressed. Especially if you are familiar with a song live, if you know the beginning is soft but on a recording it is loud, then the volume has been raised for the beginning. Basically the whole song is at one volume level and any changes in volume during the song (dynamics) are gone.
