The author of this article created a small app that allows users to send and receive tweets using SMS and Twilio. The app was built in 62 lines of Ruby, and the first part of the blog post series covers setting up the application, including creating a Gemfile, configuring environment variables, and defining a Twitter client. The app uses Sinatra as its framework, Rack underpins it, and Thin is used as the web server. The author also secured the endpoint to prevent unauthorized access by checking if the SMS was sent from their phone number and using the Twilio Ruby gem's Rack middleware for authentication.