readStringUntil() reads characters from the serial buffer into a string. The function terminates if the terminator character is detected or it times out (see setTimeout()).
This function is part of the Stream class, and is called by any class that inherits from it (Wire, Serial, etc). See the Stream class main page for more information.
terminator : the character to search for (char)
The entire string read from the serial buffer, until the terminator character is detected