Headers and footers are the top and bottom sections of the document respectively. They are separate sections from the main document, and are often used to hold footnotes, page numbers, titles, and other information.
The header and footer controls are in the Header & Footer group of the Insert tab. (Insert>Header & Footer)
The Header and Footer options each contain a dropdown menu. Click one of the Built-in options to insert it, or select Edit Header (or Edit Footer) to edit the header (or footer).
The header/footer will be represented by a dotted line. While in header/footer mode (when the dotted lines are visible), you cannot edit the main text. You can go back to editing the main text by selecting the Close Header and Footer option on the right side of the ribbon.
The information in the header or footer is repeated in every header or footer in the document.