A prefix is a small sub-unit of language. It may be added at the front of any of many other words to indicate a change of meaning. This is related to the word's original meaning, for example the prefix 'un-' gives the negative to the original: 'unnatural' means 'not natural'. The word 'prefix' itself contains the prefix 'pre-', which means 'before' added to 'fix', which shows that a prefix is 'something fixed before' another word.
Prefixes make one of a small group of bound morphemes which cannot normally be used on their own, but must be attached to other words. Suffixes form another such group. The general name for the whole group is affix. (Note the two prefixes here: 'su-' means 'after', and 'a-' here is the more general 'to'. An affix is any such morpheme 'fixed to' a word. In English, the important affixes are prefixed or suffixed; but there is a category of infixes, which is larger in soime other lnguages. You may now notice from the prefix 'in-' that an 'infix' is a sub-unit that is inserted into the middle of a word.