In everyday collecting, a first edition means the first printing of a book's first appearance, from the first setting of type. Strictly, an edition is every copy printed from one setting of type, so a first edition can include several printings; collectors usually want only the first printing of it, often called the first impression.
How to spot it
Start at the copyright page. Look for a statement such as "First published in 1965", "First edition" or "First printing", and for a number line that includes a 1. Check that the date on the title page agrees, and that there is no list of later printings. Publishers had their own conventions, and some never marked first editions at all, so a publisher's rules matter as much as the words on the page. Later editions often say "Second edition" or "Revised edition".
What it does to value
Almost all of a collectable book's value usually sits in the first printing of the first edition, in its original jacket where it had one. Later printings of the same edition are typically worth much less, and later editions less still.
Read how to identify a first edition, decode the copyright page with the number line decoder, or scan the barcode with the book scanner to find the edition.