Flight controller board - arduino Compatible?

what is it mean arduino Compatible?
i want to buy APM 2.6 flight controller board with the Arduino Mega microcontroller and its arduino Compatible.
does it mean the board is also functions as arduino or it just mean i can run it with the arduino software but with special library?
i can just write "digitalWrite(13,HIGH)" and it work? or im going to have a lot of complications?

The APM 2.6 autopilot was developed using the Arduino environment and the code is open source. So, if you want to, you can modify the program code using the usual Arduino development system. It is a large program.