A suffix is an affix that follows the morphemes to which it can attach. Look up affix in Wiktionary, the free dictionary. ... In morpheme-based morphology, a morpheme is the smallest lingual unit that carries a semantic interpretation. ...

In plain English, it's a small addition at the end of a word (or part of word).

An example from colloquial (British) English would be the word "soonish", where the "ish" modifies the "soon", meaning "soon, but not all that soon".