readStringUntil() reads characters from the serial buffer into a String. The function terminates if it times out (see setTimeout()).

Serial.readStringUntil() inherits from the Stream utility class.

Serial.readStringUntil(terminator)

Serial: serial port object. See the list of available serial ports for each board on the Serial main page.
terminator: the character to search for. Allowed data types: char.

The entire String read from the serial buffer, up to the terminator character. If the terminator character can’t be found, or if there is no data before the terminator character, it will return NULL.

The terminator character is discarded from the serial buffer. If the terminator character can’t be found, all read characters will be discarded.