The RSS widget shows the recent posts from another site, using that sites RSS feed.
RSS (Really Simple Syndication) is a way to get updates from websites without having to visit them. It delivers new content like news articles, blog posts, or videos directly to you in one place, through an app or reader, so you can easily stay informed about what’s new.
Most blogs and news sites have RSS feeds. Glow blogs have RSS feeds, the URLs are the address of the site with feed appended to it. For example the address of this sites is:
https://blogs.glowscotland.org.uk/glowblogs/widgets/
So the RSS feed address is:
https://blogs.glowscotland.org.uk/glowblogs/widgets/feed/
On the example on the right on this page we used the feed from the main help blog:
https://blogs.glowscotland.org.uk/glowblogs/glowbloghelp/feed/
The RSS widget is available as a block in the Block editor: