The denouement, also known as the "falling action" or, potentially, "resolution" (depending on what approach you use), is everything that happens after the climax. It's where any remaining conflict or tension gets addressed and dissipated, where important loose ends get tied up, and where change (or the lack thereof) gets validated. It's also usually where a new "normal" is established, or at least hinted at.
People mistakenly think denouements are all about ending the story quickly. While some writers certainly approach them that way, it's more important to address what I just mentioned if you want a satisfying denouement.
Recently, a commenter asked me about the length of denouements, and even though I responded, my mind has still been mulling over the topic. Generally, my rule of thumb is this: Keep them short enough to stay interesting but long enough to cover the important parts. (The important parts being what I listed in the first paragraph.)
