An "ONSET" is a vocal term that refers to the beginning of a vocal phrase or sung sound. A common habit of singers is to start vowels with a harsh Onset, known as a "glottal onset" or "glottal attack". This is not wrong or harmful in itself. The problem is doing it too often or too harshly. The first step in changing this is to become aware that it's a habit. Record yourself and listen to your phrases beginning with vowels. If harsh Onsets are a problem, try placing a gentle inaudible "H" in front of the word or phrase. This begins your vocalization in a much smoother way. There are times when you might choose a harsh glottal attack as a stylistic device, like in a rock song where you're seeking an edgier sound. Even still, you'll benefit from knowing how to do a balanced Onset, as you'll be able to finely tune your vocal fold compression and breath!