Complex sentence: The defining characteristic of a close vowel is that the tongue is positioned as close as possible to the roof of the mouth without creating a constriction that would be classified as a consonant.
Split into simpler sentences: The defining characteristic of a close vowel is that the tongue is positioned as close as possible to the roof of the mouth as it can be without creating a constriction. A constriction would produce a sound that would be classified as a consonant.