Frontness
What part of the tongue is involved; that is, what part is raised or lowered? Is it the part close to the front of the mouth, the back, or the center? These are ever-so-cleverly classified as “Front,” “Central,” and “Back.” Front vowels are heard in “Bee,” “Snake,” and “Cat.” Back vowels are heard in “You,” “Show,” and “Father.” English has only two (some would lump them together and say only one) Mid vowels.
Essentially, it’s the same sound but is classified as to whether it’s used in a stressed or unstressed syllable. For stressed syllables, the Mid vowel is that which you hear in “Shut*Up.” The same sound, in an unstressed syllable, is the famous “Schwa Sound,” which all English vowels very sneakily mutate into whenever you don’t bother to put syllable stress on them. It’s the sound of the first syllable in “about,” for one example.