A morpheme is the smallest semantically meaningful unit in a language. When you break a word up into segments, it's the list of parts that have combined together to create that word.

A word is at least one morpheme, but a morpheme may be less than a word; morphemes need not be free-standing (they could be a bound morpheme.

E.g. "indistinguishable" can be broken down into "in" + "distinguish" + "able".