A preamble is an introduction to a document that describes the documents purpose. The word comes from "pre" meaning "before" and "amble" meaning walk. A preamble can contain facts about the document. The preamble of a law is not part of the law but explains its purpose. The preamble of a book is called a preface. The preamble of a musical composition is a prelude or overture. The preamble of a Wikipedia article walks before the sections, and is called the lede or introduction.
A data packet has a preamble which the system needs but which goes before the data that the user will use.
One of the world's most famous preambles is the one in the United States Declaration of Independence in 1776:
We hold these truths to be self-evident, that all men are created equal, that they are endowed by their Creator with certain unalienable Rights, that among these are Life, Liberty and the pursuit of Happiness. — That to secure these rights, Governments are instituted among Men, deriving their just powers from the consent of the governed, — That whenever any Form of Government becomes destructive of these ends, it is the Right of the People to alter or to abolish it, and to institute new Government, laying its foundation on such principles and organizing its powers in such form, as to them shall seem most likely to effect their Safety and Happiness. Prudence, indeed, will dictate that Governments long established should not be changed for light and transient causes; and accordingly all experience hath shewn that mankind are more disposed to suffer, while evils are sufferable than to right themselves by abolishing the forms to which they are accustomed. But when a long train of abuses and usurpations, pursuing invariably the same Object evinces a design to reduce them under absolute Despotism, it is their right, it is their duty, to throw off such Government, and to provide new Guards for their future security.