In phonetics, this refers to a stop at the end of a word which is held at the point of articulation. Usually, a stop is released into a vowel, another consonant, aspiration, glottalization, etc. In this case, it is released into nothing, but just held. In English, this is often done with /p/,/b/,/t/, and /d/ at the end of words.
In English, this convention is not distinctive, that is, if you say 'top' with an unreleased /p/, an aspirated /ph/, or even a glottalized /p'/, it will be perceived as the same sound. But in some languages, such as Arabic, this can be the difference between the meanings of two different words. This happens when a shadda is placed at the end of a word, which stands for consonant lengthening.