ALAA72:
what is this github file all about?
It's not a file, it's an Arduino library - you can even download it in the Arduino IDE "Libraries Manager".
Basically, one Arduino sends data to the library, the library "packetizes" the data and sends it via USART to another Arduino running the same version of the library. The receiving Arduino then uses the library to "depacketize" the data and allow the main Arduino code to access the data received.
The readme explains the library and it's example in as much depth as needed (if you disagree, let me know and I can edit it).