The vocal cords, also known as vocal folds, are composed of twin infoldings of mucous membrane stretched horizontally across the human larynx. They vibrate, modulating the flow of air being expelled from the lungs during phonation.
Open during breathing, the folds are controlled via the arytenoid cartilages for speech or singing. They are white because of poor blood circulation.
The folds vibrate when they are closed to obstruct the airflow through the glottis, the space between the folds: they are forced open by increased air pressure in the lungs, and closed again as the air rushes past the folds, lowering the pressure (Bernoulli's principle). A person's voice pitch is determined by the resonant frequency of the vocal folds. In an adult male this frequency averages about 125 Hz, adult females around 210, in children the frequency is over 300 Hz.
The glottal stop is the sound made when the vocal cords are pressed together to stop the flow of air and then released, and is the sound in the middle of the interjection uh-oh.
In Dutch, the glottal stop is not phonemic, but it is inserted in multi-morphemic words before morphemes that begin with a vowel, for example beamen ("to endorse"), where the glottal stop may be inserted after the prefix "be-".
The fact that the glottal stop does not change the main meaning of the word is evidenced by its (variable) occurrence in forms where it is historically secondary, and mainly concentrates the form in the information structure of the sentence, e.g.
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