In simplest terms, a collection is when you take different works that carry a Creative Commons license and arrange them together into a new work, but the constituent parts remain distinct from one another. Perhaps the easiest way to think of a collection is as an anthology: a curation of different works that are redistributed in this new package. A more fun metaphor is to think of a collection is as a TV dinner: different dishes served together on a tray that keeps each dish separated from the other.

When you create a project like this, you can still add your own content, which you can also license as you desire. For example, you might add an introductory chapter and create a cover for the work.

The benefit of putting together a collection is that you can include content that carries any Creative Commons license or items in the Public Domain. Most notably, your work may also include content that carries the most restrictive CC condition of NonDerivative (ND). This is because a collection is by nature not a derivative work (see the Remix tab for more on what constitutes a derivative work).

However, with a collection you do still need to be mindful of the license conditions of the various included works when applying a license to the collection as a whole. In particular, if any of the included works contain ShareAlike (SA) and/or NonCommercial (NC) conditions, you will need to ensure your license for the collection is compatible. So, if there are any works with an SA condition, you have to also release your work with a SA condition. And if there are any works with NC condition, you too must release the collection with an NC condition in the license.