Adding a Rich Website Summary (RSS) feed to a Website Builder page lets you display another Website's blog posts as they're updated. You also can use it to display posts from a blog of your own that's hosted elsewhere. (You cannot use Website Builder itself to create or publish a blog.)
This feature uses an API (application program interface) that Google stopped supporting in late 2015. That means the feed widget is no longer reliable: It may work one day and not another.
By default, the Blog or RSS Feed field uses a sample RSS feed address. Replace it with the feed address of the Website you want displayed. They typically end with /feed/.