Prefixes are bound morphs that can appear in beginning of the compounds, mostly forms of nominals. Finnish does not have almost any real prefix morphemes.
Prefixes examples:
alabanaani:ala+N+Pref#banaani+N+Sg+Nom (Eng. # lower banana)
erikoistyyppi:erikois+N+Pref#tyyppi+N+Sg+Nom (Eng. # special guy)