What do the charactors " '/2' " mean in the variable declaration?
The '\2' and '\3' are characters with octal code 2 and 3. Which is nothing more then 2 and 3 in octal and in decimal. The author could simply write const byte STX = 2; const byte ETX = 3;.

Characters with ASCII code 2 and 3 are control characters STX (start of text) and ETX (end of text). Control characters are invisible characters used to control the terminal or as mark characters in communication.