A prefix is a set of letters added to the beginning of a word, that changes what it means. For example, the prefix un- usually changes the original word to the opposite meaning (happy/unhappy).
There are many common prefixes. This doesn’t work for all words, but if you learn how these prefixes are used, you can increase your vocabulary more quickly. (unlucky; unzip; uncut; unopened; unwrap; unknown; unlock; etc.)
Here are some of the most common prefixes:
Finally, here’s a larger list of 50 common prefixes.