The glottis is the space between the vocal cords, and when this is blocked (or stopped), a glottal stop is produced.
It can be hard for English speakers to identify this sound, despite the fact that it is quite common in English, often used to replace a ‘t’ sound in words like ‘football’ or ‘not yet’. It is stereotypically characteristic of Cockney English words like ‘bottle’. It is also used in expressions like ‘uh oh!’
While in English this sound doesn’t change the meaning of any words, in many languages it does. For example in Kunwinjku (where this sound is spelled ‘h’) it can change aspects of the verb:
ngare = I go, I will go
ngahre = I am going