Resonance
As you can see on the above illustration the first place the sound goes once it has been created in the larynx (vocal folds vibrating), is the pharynx, which can be broken down further into three parts, the Laryngopharynx, Oropharynx and the Nasopharynx (which can be closed at will), the pharynx is the largest of the resonators, and thus influences the sound the most, if the pharynx (throat) is strained when you sing then the walls of the pharynx will be harder and as a result less of the overtones are absorbed so the sound that comes out is more strident or shrill, if the throat is allowed to be relaxed (the sensation is in singing as if you are not using your throat at all) then more of the unpleasant overtones are absorbed and the sound will be more pleasant, if you over do this and darken the sound to much then the sound could be described as sounding swallowed, and it will have no focus, so as with everything in life there must be balance, in order to get the best sound.
After the throat (Pharynx) the sound travels through the Oral cavity (mouth) and here more resonance is added (if you hum then you should feel your lips vibrating, this is oral resonance).
How much nasal resonance is in the voice is a matter of taste, but the sound of nasal resonance in its purist form is a honky sound, just pinch your nose and talk and you will see want I mean! In order for nasal resonance to be present in the voice the soft palate (velum) must not be high but low, so for a clear voice the velum should be high so as to minimize nasal resonance, by blocking the path to the nasal cavity.
Resonance is like a vocal equalizer it can be blended so as to lighten or darken the sound, depending on what the dynamics of the lyric demand.