A prefix is affix that's added in front of a word and often changes the meaning of it. Common prefixes include re-, un-, and mis-.
Lots of words have a prefix, and it's sometimes easier to figure out the meaning of those words if you know the meaning of the prefix. For example, the super- means "above" or "over"; hypo- and infra- mean "under." Prefix itself has a prefix: pre-, which means "before." In fact, the Latin root of prefix is praefixus, meaning "fixed in front." (Note our word's opposite here, suffix, which describes what is fixed at the back of a word.)