Energia provides a convenient library of built-in functions we can use in our programs. If you click on the Help button in Energia, it will bring up the Arduino Language Reference page, since Energia is based on the older Arduino platform. That is where you will find the documentation for many of the functions we have used in the examples in this book, such as pinMode(), digitalWrite(), analogRead(), etc.
Most of those functions are known to the compiler already. Others, such as the Servo class, are in extra libraries, and we need to include a header file before we use them. The Arduino Language Reference page you get when you click on the Help button in Energia has a link to the library page, where several libraries are described. In addition, you can create your own libraries.