The glottal stop is the sound produced by blocking off ('stopping') the airflow of speech in the throat ('glottis'). It is a recognised letter in some languages, such as Arabic. There is a phonetic symbol for it - IPA: /ʔ/.
In English, it is only used informally, as a different way of realising a 't'. In London, for example, local speakers may be represented as saying "wa'er" instead of 'water'; in Glasgow, "bu'er" is common for 'butter'.
It is common in many urban accents in Britain; but it is regarded in academic circles as uneducated (or of low status). Try, in formal contexts, to pronounce 't' clearly rather than the glottal stop.