Previewing an email allows you to see what your contacts will receive. If you're using Liquid or any other dynamic content, such as Cart Contents, previewing is the best way to ensure that your email will look as desired.
To preview an email, you can click on "Preview" on the email screen. In the preview window, you can toggle between Desktop and Mobile views.
You can also preview your email directly from the email editor. When editing an email, click on "Actions" at the top right, then on "Preview".