The book refers to a exmaple.js file in section 3.4 on running a MongoDB example. It is not clear exactly what this line does in page 27
mongo localhost/tweetdata path/to/example.js
path/to/example.js is a dummy file location for you to replace with either: 1) one of the book's examples, or 2) a script you wrote yourself. That path is simply a placeholder added for completeness.