This sound is also called the glottal stop. In the International Phonetic Alphabet it is represented by ‘[ʔ].’ It occurs in many world languages, including some dialects of English. The approximate pronunciation is that of the small sound between: ‘uh-oh.’ More exactly:
Voiceless: the larynx should not rumble during articulation.
Glottal: the sound should come from the glottis.
Plosive: sound produced by stopping airflow and suddenly releasing it.
Link to site of spoken samples of all IPA symbols: IPA Tutorial