This project uses 'Keyboard' function in Arduino. Keyboard function is only supported in Arduino Due and Leonardo. So when you connect your arduino to your laptop or computer, you can write a keyboard function to send a particular key or mouse instructions. I have hard coded few sentences and words which I use regularly to do SEO in my blog. Its an awesome tool when you have repeated task.
This video explains it all. You can also look into instructions for steps to create this project.