Companion Computers can be used to interface and communicate with ArduPilot on a flight controller using the MAVLink protocol. By doing this your companion computer gets all the MAVLink data produced by the autopilot (including GPS data) and can use it to make intelligent decisions during flight. For example, “take a photo when the vehicle is at these GPS co-ordinates”, gather and pre-process information from advanced sensors or actuate lights, auxiliary servos or any other interfaces.
There are two major parts to Companion Computers - hardware and software.
The Companion Computer hardware refers to the specific computer hardware being used. This is typically a small ARM-based Single Board Computer. Specific tutorials for popular Companion Computer hardware are listed below.
The Companion Computer software refers to the programs and tools that run on the Companion Computer. They will take in MAVLink telemetry from the Flight Controller and can route and process the telemetry data. Specific tutorials for popular Companion Computer software tools/suites are listed below.
A number of vendors also offer turnkey systems. See the below link for details.