A preamble is a brief introduction to a speech, like the Preamble to the Constitution that starts out "We the People of the United States, in Order to form a more perfect Union...do ordain and establish this Constitution."
Preamble comes from the Latin praeambulus which means "walking before." And that's what a preamble does — it "walks" before a speech, often explaining what's coming. It's like the White Rabbit introducing the Queen of Hearts in Alice in Wonderland. Since it goes before a speech, think of it as a pre-ramble. A preamble is usually used for formal documents; you wouldn't include one in a text to your best friend.