What's this supposed to mean? Should I be worried?
(Most likely mentioned before, but couldn't find it on the site, so if there's a link to that, please guide me to it)
It means the connection is not using HTTPS (for which the site owner needs to obtain a TLS cerificate), see f.e. https://www.bluehost.com/blog/how-to-fix-the-https-not-secure-message-in-chrome/
If you are the site owner, you can buy such a certificate for your domain, or use a free service like LetsEncrypt.
Thanks, will check it out :) So, it only happens in chrome?
I'm not sure and I can't test right now; I believe Chrome was the first to show the warning, but I think the other browser vendors followed.
You can test yourself with sites like http://neverssl.com
It happens in other browsers. I don't see it in Firefox because there's a config option to suppress the warning. It's really very annoying, but otherwise not a major concern.
Maybe for others some info about how the issue was solved...
My server (One.com) has SSL activated for all the sites they host. but I had to add
#Rewrite everything to https
RewriteEngine On
RewriteCond %{HTTPS} !=on
RewriteRule ^(.*)$ https://%{HTTP_HOST}%{REQUEST_URI} [L,R=301]
in the .htaccess file. That still gave me the "not secure", but after changing the path of my banner picture from http://xxx to https://xxx, the 'problem' was solved