Besides the availability of a device driver, some hardware also requires so-called firmware or microcode to be loaded into the device before it can become operational. This is most common for network interface cards (especially wireless NICs), but for example some USB devices and even some hard disk controllers also require firmware.
In most cases firmware is non-free according to the criteria used by the Debian GNU/Linux project and thus cannot be included in the main distribution or in the installation system. If the device driver itself is included in the distribution and if Debian GNU/Linux legally can distribute the firmware, it will often be available as a separate package from the non-free section of the archive.
However, this does not mean that such hardware cannot be used during an
installation. Starting with Debian GNU/Linux 5.0,
debian-installer supports loading
firmware files or packages containing firmware from a removable medium,
such as a floppy disk or USB stick.
See Section 6.4, “Loading Missing Firmware” for detailed information on how to
load firmware files or packages during the installation.