For questions pertaining to the Arduino microcontroller and it's integration with Raspberry Pi projects. Questions about Arduino only are off-topic and can be asked at Arduino Stack Exchange.
The Arduino is a single-board microcontroller, designed to make the use of electronics on technology projects easier. The Arduino is visually similar to the Raspberry Pi, but it is focused more on using electronics in programming, than just programming like the Raspberry Pi is (although GPIO may be used for controlling electronics).
If you have a question about Arduino only, visit our sister site Arduino Stack Exchange.