pre comes from the Latin word prae meaning before. This prefix is used to mean 'before' in time, place, order, degree or importance.
Here is a selection of words using this prefix.
preamble (noun) - a preliminary statement.
preamplifier / preamp (noun) - an electronic device that amplifiers a weak signal (usually a microphone or record-player) to a level suitable for a power amplifier.
pre-arrange (verb) - arrange beforehand.
precaution (n) action taken beforehand to avoid risk or ensure a good result.
precede (verb) - come or go before in time, order or importance.
precedent (noun) - a legal term referring to a previous case taken as a guide for subsequent cases or justification. In more common use, a precedent is an action or behaviour that indicates a possible future action. For example, "Are you setting a precedent by arriving late this morning?".
prefix (noun) - a verbal element placed at the beginning of a word to qualify its meaning. For example, the opposite of a conformist is a non-conformist. In the previous sentence, non is the prefix.
Misuse of the prefix in modern language.
I have noticed several misuses of pre when signwriters and copyeditors coin new words.
Medical insurance companies ask if you have have any pre-existing conditions. Surely they are asking what conditions you currently have that could affect the insurance policy?
Pre-exist is not in my dictionary. What is meant by pre-existing? Is it something that existed before but doesn't exist now?
When you are confronted by the pre-existing word, please ask the speaker if they meant existing.
Another popular word is pre-order. Retail advertising implores me to "pre-order my copy NOW!". Why can't I just order the product? Pre-ordering implies I place the order previous to the release of a product. Surely the phrase "order your copy NOW" carries the same meaning without the unnecessary burden of a prefix?