From ArchWiki

[edit] Definition

A development repository makes preview, alpha, beta, and pre-release software available for users to download, install, and use. As opposed to a standard repository, which only provides stable versions of software, this could be considered extremely bleeding edge.

[edit] Warning

This software is considered unstable by the developers. It's possible that development software will do horrible, evil things to your files, your computer, and your life. It also may not work as expected. By installing software from development repositories, you acknowledge, and accept this warning. The repository maintainer only provides the packages, (s)he cannot guarantee that the packages will work.