
Nofollow is just one of those HTML attributes that can be added to links to discourage the comments spam. It is used with the rel=" " code inside a link. When Google and other search engines sees that specific attribute (rel="nofollow") on hyperlinks the alghoritms won't give any credit to those links when we rank websites in our search results.
For example, a link with nofollow attribute looks like this <a href="https://another-website.com/its-article" rel="nofollow">Article title/a>.
As a general SEO best practice you should add nofollow to all external websites that you don’t trust or you don't give credit when citing them as sources for the news you are writing about. If you have two or more websites you can send links to them but you do it carefully, you don't want search engines like Google you are creating a network of websites just to generate backlink and increase the authority for your websites - it's probably as bad as paying for those backlink (you shouldn't buy them, the increase should be natural not generated).