An external webview is when a link to a webpage in an item of an app takes the user, when clicked on, outside of the app. For example: If an external webview was set up in the app, if you click on the link to www.externalwebview.com within the app, it will take you to the Safari app and then open the webpage. To get back to the app, you will have to go back out to the menu and click on the app.
Apple has requirements and restrictions on the content that can be put into apps. If there is in app purchasing or the ability to give money through an app, Apple requires that they receive a percentage of the money given. It is similar to a charge or fee. In order to prevent this from happening, external webview takes the ability to give or purchase outside of the app, thus removing that fee.
Add the website property to the item you created. Type in the url in the bottom field and the name you want displayed into the top slot. On the right side of the property there is a check box that says, "Open Externally." Check this box to display the website in a browser outside of the app.
When to use Open Externally for websites:
Donations or Mobile Giving have to be opened externally. If donations happen within the app, Apple will either reject the app from the App store or require a percentage of each donation. In order to avoid this, open these kinds of links externally.
Similar to donations, Apple requires a percentage of all in-app purchases. An app will get rejected by Apple if it includes purchases within the app.