In the Larynx, the vocal folds open and close to let air pass and to keep food from falling down the Trachea.
Stretched across the opening of the cricoid cartilage are the two vocal folds made mostly of connective tissue. They are always closed in front at the point where they meet, but swing open and shut from the other rear.
The rapidity with which these folds can open and close (an alternating movement called diodochokenesis) is unparalleled in the body. When we swallow, the vocal folds close snuggly. There is actually another point of closure just above the folds called the False Vocal Folds.
NOTES: More information on the Larynx.