Sidenote is a browser extension that allows users to engage in community conversations while browsing any webpage. By adding a sidebar to the browser, it enables users to see the thoughts, questions, and comments of others, enhancing their browsing experience. The main advantage of this extension is its ability to interact with any webpage, allowing users to instantly partake in discussions and gather information. Background information indicates that Sidenote aims to improve users' web experience through the power of community.