The Preview feature allows you to see your card or event from the viewers perspective before you publish it live to the app.
To do this, simply look for the 'Preview Changes' button. It can be found within the Publish Widget of the card editor.
Once clicked on, a preview will be opened in a new tab.
Note: If you have the Web App enabled in your App Settings and have a passcode set, you may be asked for this upon previewing content.