A prefix is a letter or group of letters added to the beginning of a word. Knowing what a prefix means can help us to understand new words.
If a person is a co-author of a book it means they have written it with someone else.
(We use a hyphen because without it the word coauthor would have three vowels together and would be difficult to read.)
We use antiseptic cream to stop a wound from becoming septic (infected).