Embouchure - how you shop your mouth - affects the sound of your voice. As you smile it tends to give your voice a brighter sound and as you pucker, it gives your voice more flexibility and a darker sound.
This exercise uses three vowels sounds sung on an octave interval. First sing "oo," "ie," then "eh." The "oo" helps you get into a pucker shape, the "ie" is open and bright, and the "eh" is neutral.