DCS-BIOS connects custom-built control panels powered by Arduino microcontroller boards to the flight simulator
It can also be used by other programs to interface with DCS.
The DCS-BIOS documentation is available online at
If you are a new user, start with the first section,
DCS-BIOS YouTube channel
contains a few tutorials which were recorded for a very early version, so some of the specifics are out of date, but they are still useful to understand what you can do with DCS-BIOS.