A header or footer is content that repeats automatically on every page — a document title, a date, or a page number.
Insert tab → Header or Footer, or simply double-click near the top or bottom of a page to start editing it directly.
Insert → Page Number, then choose a position. Word numbers every page automatically as content is added — including any pages inserted later, no renumbering by hand.
Header & Footer tools → Options offers Different First Page (a cover page with no header) and Different Odd & Even Pages (page numbers on alternating sides, the way a printed book positions them) — both common requirements for a formal report or thesis.