A prefix is a word part that is added to the beginning of a word to change, or add to its meaning [what a word represents]. For example, the prefix un- (meaning “not”) is added to the word happy to make the word unhappy. [Happy means “feeling good”, but unhappy means “not feeling good”.]
Very Simple Definition:
A prefix is a part added to the start of a word to change what it means. For example, when we add the prefix “un-” to “happy”, it becomes “unhappy”.