Diction is the accent, inflection, and pronunciation of sounds that form words. We say someone has “good diction” when we can easily understand what they are saying. In the same way, a singer has good diction when the words he or she sings are understood. When we sing we are pronouncing lyrics; this is why learning to sing words clearly is called lyric diction.
Singing words feels different from saying them. Learning how to pronounce words well while singing will improve voice quality by enabling the singer to produce an open relaxed sound.