I have seen many tutorials on programming an Arduino to control a HC-05 bluetooth module. In that I did not understand the term called 'flag'. What does that mean?
I've always used flag to indicate a boolean logic value. I would also suggest this is an archaic term.
Was going to come up with my own explanation but found the following at In C language, what does flag mean?:
A 'flag' is a variable that is supposed to signalize something. Think about the word itself - flags are used in sending signals. In our world of programming a basic boolean flag can either be "up" or "down", either true or false.