This app stores all data in your browser. When you enter new contacts, they are saved in the browser. If you switch to a different browser or access this page on a different computer or phone, they will be gone. If someone else uses your computer and opens your browser to this page, they will be displayed to that person. So... don't use this app on a public computer. If you care about privacy, you may also want to ensure you disk or device is encrypted.
To send sms messages, you must register for a Twilio account and fill in the information in the Twilio configuration section.