If the prefix means or what does really mean?

"Prefix" means "placed before." It refers to a word element that is added to the beginning of a word to modify its meaning.

For example:

* un- is a prefix that means "not" or "opposite of".

* pre- is a prefix that means "before" or "prior to".

* re- is a prefix that means "again" or "back".

Therefore, when we talk about the "prefix" of a word, we are talking about the part that comes before the main root of the word.