Preview mode shows you what your website will look like to visitors before publishing the changes you have made. To preview your website, simply click on the Preview button in your Greenhouse in the top right corner.
If you haven't launched your site yet, you will see the Preview button under Continue Site Setup:
If you have launched your site, you will see the Preview button under Publish Changes:
Preview on Mobile
To preview your site on your mobile device or tablet, you will need to click "Mobile Preview" directly to the left of the "Preview" button.