Preview allows users to see how their code looks like when it's running. It is a must-have feature in most applications. To open a preview click on a Preview button in the header and choose the port you are using. For example, if your application uses port 3000, click on http://0.0.0.0:3000 link.
In most cases, preview works without a need to do any modification but don't panic if it initially doesn't in yours.