A folio has several related meanings primarily connected to books and printing:

1. A folio is a large sheet of paper folded once to create two leaves or four pages in a book. This method produces a book or pamphlet where each leaf is half the size of the original sheet. This folding and assembling technique was common in historical book production. Famous examples include the Gutenberg Bible and Shakespeare's First Folio.
2. It is also a term used to describe a page or leaf in manuscripts and books, where each folio has two sides: the recto (right-hand page) and verso (left-hand page). Folio numbers can be used to reference specific leaves instead of pages.
3. Additionally, folio refers to the approximate size of a book, typically about 15 inches tall, larger than quarto and octavo formats, often used for special editions or art books.

In summary, a folio can mean a physical sheet folded in bookmaking, a leaf or page in a manuscript or book, or a book size/format designation. It also can mean a collection of loose, sequenced prints like an artist's presentation, resembling but not bound as a book. The term originates from Latin "folium" meaning "leaf" and is fundamental in book publishing and printing terminology. This covers the main meanings and contexts where "folio" is used in books, manuscripts, and printing. Let me know if you want details on any particular sense.