PS: I suppose I should offer an explanation of my own, so:
A "non reference-level" system will only allow you to hear a great deal less than 100% of the information that is contained in your source, ie: CD, MP3 download, Vinyl disc etc. You could say as little as less than 50% will get across to your ears.
A "reference level" system will relay at least 90% of what is available from your information source, also presenting it in a musical way with proper pitch, pace, rhythm and timing.
These parameters would apply to using headphones as well, of course.
The percentages I use are approximate only, just used to illustrate the situation.
Hope that helps. 😀