What does the prefix mean?

The word "prefix" means "something added to the front". In the context of language, a prefix is an affix that is added to the beginning of a word, root, or stem to form a new word. Prefixes can change the meaning of the word, such as in the case of the prefix "un-" in the word "unhappy", or they can change the part of speech of the word, such as in the case of the prefix "re-" in the word "reheat".