SDIO is an interface designed as an extension for the existing SD card standard, to allow connecting different peripherals to the host with the standard SD controller. Peripherals currently sold at the general market include WLAN/BT modules, cameras, fingerprint readers, barcode scanners.
Besides that, some embedded boards offer the integrated SDIO controller with some peripherals connected to it. For example, GlobalScale DreamPlug has an internal SDIO slot with the Marvell WLAN/Bluetooth module.
FreeBSD lacks SDIO support and is not able to talk to the peripheral devices behind SDIO. Work is going on to create a driver for the FreeBSD, based on DreamPlug (h/w) and FreeBSD-HEAD.
The source code is kept here: https://github.com/kibab/freebsd/tree/kibab-dplug
Wikipedia article about SD and SDIO: http://en.wikipedia.org/wiki/Secure_Digital#SDIO
SDIO specification: https://www.sdcard.org/developers/overview/sdio/sdio_spec/