Reference → Serial →

Serial.write()

Writes binary data to the serial port. This data is sent as a byte or series of bytes; to send the characters representing the digits of a number use the Serial.print() function instead. Serial.write(val)
Serial.write(str)
Serial.write(buf, len) Z-Uno board also supports Serial0 and Serial1. val a value to send as a single byte
str a value to send as a string of bytes
buf a value to send a array of bytes len the length of the buffer write() will return the number of bytes written, though reading that number is optional. Data type: size_t.

        void setup(){
  Serial.begin();
}

void loop(){
  Serial.write(45); // send a byte with the value 45

  int bytesSent = Serial.write("hello"); // send the string "hello" and return the length of the string.
}