An ELT Glossary : Glottal Stops/Plosives

A glottal stop (or glottal plosive) is formed by briefly closing the vocal cords (the glottis) allowing air to build up behind them and then releasing it. It is not a phoneme of English (ie never makes a meaning difference) but is frequent  in connected speech. It occurs, for example, in the expression uh-oh, between the two syllables.

The IPA (International Phonetic Association) symbol for the glottal stop is [ʔ].

The glottal stop is also a common allophone of the unvoiced alveolar plosive /t/, particularly in certain varieties of English such as London ("Cockney") and Estuary English.
If you're not sure what it sounds like you can hear it here