It means the site isn't running https. https used to only be used for banking, shopping, etc., but in the last year or two, more and more sites have switched their entire site over to it. A lot of forums, like Orangemane and other non football forums aren't https, but I do think it would be good. I tried to implement it a few months ago, but it wasn't working and is going to take some work to implement.
I'm going to try and get it going in the near future.
In essence, it means if someone between you and the server are listening in, then they can see your password. It's the same way that ANY email you send is plain text and anyone between you and the recipient and any admin on the email server at both ends, can view your email contents. It's why you should never, ever, ever, never, put anything sensitive in emails.