In phonetics, the glottal stop is a guttural sound that is made when the glottal folds are pressed together.
In most languages in which it is found it precedes an initial vowel and usually is not rendered in writing in such cases. Sometimes it occurs in the middle of a word between two vowels. In English it is used in several accents (eg. Cockney) as a replacement for the phoneme /t/. (Thus a phrase like "Luton Airport" would be rendered as "Lu'on Airpor'".)
In many other languages, it is a full phoneme by itself. In these cases, it is sometimes written as an opening single quote ‘, as in Hawai‘ian, where it is called ‘okina. Other examples of language using phonemic glottal stop are Nahuatl and many other Native American languages, Samoan, Hebrew, Arabic, and Japanese.
A German language example of the glottal stop is "Beamter". A fairly universal English language example would be "uh-uh". English speakers often have difficulty perceiving this sound, since it is either "invisible" or an allophone of another phoneme.