User registration

That means that you still have user registration enabled via Settings -> General in your blog’s Dashboard.

If you don’t need anyone to register for your blog, there is no need to have that enabled. Just disable ti via Settings -> General and remove any users you don’t recognize via the Users section of your blog’s Dashboard.

Regarding newsletters, there are several plugins for this: https://www.ads-software.com/plugins/tags/newsletter

Hi, ok I see that but what does it mean?
Do they get an email from my blog every time there’s a post or what?

It doesn’t mean anything unless you have provided functionality for it. WordPress provides the registration system, you have to provide plugins that make use of it, like subscription plugins, forums, etc.

The only thing WordPress can use the general registration system for out of the box is requiring registration to comment.

If you don’t require that, and you haven’t added any plugins to make use of registration, you might as well turn it off via Settings -> General in your blog’s Dashboard.

I just downloaded the newsletter manager plugin so that means I can use the registration to capture emails for the newsletter?
Sorry, it’s the first time I’m using something like this
Thanks!

I don’t think so, that plugin appears to have its own registration system.

In general, it’s just best to disable registration via Settings -> General, delete any current Users you are unfamiliar with now, and don’t enable registration again until you specifically encounter something that makes use of it.