Hiya! 
Very basically, “not secure” means that the data you’re looking at isn’t encrypted. It’s just sent plainly from the server right to you. Years ago, everything used to be “http” (unsecured), and now lots of things use “https” (s = secured). I didn’t notice it before, but it’s possible that something happened to their security key system (perhaps it expired or something).
Just because it’s not secure doesn’t mean it has a virus, it just means that if someone tries hard enough, they can read any data that you submit through it, or can tell that you’re looking at it.
So I’d personally avoid anything that involves sensitive info (bank info, etc) until it goes away, but otherwise I wouldn’t worry too much. 
Undoubtedly you’ll get a shitton of Episode ads as the ad trackers see what you’re up to though.