This is a very technical area that is very hard to explain without being physically present with the student. Briefly, to be a good singer, you have to learn to pronounce words with clear, open,  pure vowels. For most students this means learning to say words with much more energy and vibrancy than the normal way they speak. Not only will the audience understand better what you are singing, but the sound itself will be aesthetically more pleasing, (even to someone who doesn't understand English). Our language is remarkably complicated, in terms of meanings, spelling and pronounciation, and the odds that the way you learned to speak as a child happens to be the most efficient use of the voice are fairly small. Projection is another way to approach this problem, as vowels that are forward and bright to the singer will carry well, and sound attractive (clean and open) to the listener.