It's used inflectively to make -ing words.
-ing is a morpheme that is typically acquired fairly early by English language learners and usually without any accuracy issues.
-ing is generally pronounced /-ɪŋ/. /-ɪŋ/ may be further reduced to /-ɪn/ in colloquial speech. Historically the pronunciation was /-ɪŋg/ and this is still to be found in some parts of Northern England and the English Midlands - see singer-finger split. Some speakers in Northern England devoice the /g/ to a /k/, making "something" sound like "some think".
This is a single syllable and it's never a stressed syllable.