1.
to begin with; first of all; at the outset
An adverbial phrase introducing the first thing said or done, often used to open a speech, explanation, or set of instructions.
2.
Introduction; Preface (section heading)
A standard heading at the front of a book or document, especially non-fiction, marking the introductory section. Equivalent to an English "Introduction" or "Preface."
Sense 1 is the everyday adverbial use; sense 2 is the fixed section heading, usually written in kana and set in quotation marks or as a title.