You can enable/disable the popup using the 'Enable' checkbox at the top of the section.
The 'Days until popup is displayed again' lets you set how long until the popup will show again. Set it to 'Test mode' to constantly show the popup. This shouldn't be used for live stores.
To set an image, you will need to tick the 'Enable image' box, and upload an image from the selection box below.
You can also set 'Heading text' and 'Description text', you can add text, which can include bold and italic using the 'Text' box.
The 'Enable social icons' checkbox shows any social networks that have been entered in the 'Social media' section of 'General settings'.
The 'Enable newsletter signup' shows the newsletter signup form, which will save all email addresses to your Shopify customers section. To add in support for a 3rd party service (like MailChimp or Constant Contact) will need an app from the official App Store.