Espressif ESP8266 User Manual page 20

Now your output should look something like this:
If you want to learn more about the Serial object you can find more in the following link:
http://esp8266.github.io/Arduino/versions/2.3.0/doc/reference.html#serial
The exercise contains another important built-in function:
delay(1000);
This instructs the processor to wait 1000 milliseconds (1 second) before continuing with the
execution. As mentioned earlier with this command it also gives the processor time to handle
other tasks, such as receiving or sending network packets over WiFi. In this context a call to
yield() does the same as delay(0).