The adjective prefatory describes something that serves as a beginning or introduction. The president of the organization made some prefatory remarks before the main speaker at the event.
Prefatory is often used to describe an introduction to a speech, book, or other text. If you are asked to speak on a highly controversial subject, you may want to first offer some prefatory remarks that you are only offering your opinion. But at least half the audience will disagree with whatever you say anyway. Your will may have a prefatory clause explaining why you've left everything to your cat Fluffy and not to your awful siblings.