KIO (KDE Input/Output) is part of the KDE architecture. It provides access to files, web sites and other resources through a single consistent API. Applications, such as Konqueror, which are written using this framework, can operate on files stored on remote servers in exactly the same way as they operate on those stored locally, effectively making KDE network-transparent. This allows for a file browser like Konqueror to be both a highly versatile and powerful file manager as well as a web browser.
KIO slaves are programs that provide support for individual protocols (e.g. HTTP, FTP, SMB, SSH, FISH, SFTP, SVN, TAR).
The kde helpcenter app Khelp has a kioslaves section that lists the available protocols with a short description