Long vowels are vowel sounds that are longer than normal, or short, vowels. In RP English the long vowel sounds are those in 'seat', 'suit', 'sort', 'shirt' and 'start'. Phonemic symbols for long vowel sounds have a /:/ to indicate length.
The word 'kiss' has a short /i/ sound, whilst the equivalent long sound /i:/ produces the word 'keys'.
In the classroom
Differences in length in vowel sounds are difficult for many learners. One approach is to highlight the difference in minimal pairs (pairs of words that have only one difference, in this case the vowel length) such as the example above and to model the different sounds for learners to copy.