A 'security certificate' is proof that a website is actually the one you think it is. It's not a paper certificate, of course, it's digital, and based on internationally accepted standards.
Security certificates need to be renewed every year. If a website presents an out-of-date security certificate, a web browser won't allow the website to display, which is what happened in your case.
When a website presents an expired security certificate, that's usually - though not always - a sign of trouble. It
is possible that the list of valid 'certificate authorities' in your tablet is simply out of date and needs to be updated. But not very likely, because the list of valid certificate authorities (aka CAs) is regularly updated when your copy of Windows
is updated. That's why you got (valid) advice to update your copy of Windows. However, if your list of CAs was out of date, you would be having trouble with lots of websites, not only Wildblue.
It's more likely a problem on the Wildblue side of the connection. It's something Wildblue has to straighten out, in which case there isn't anything you can do about it. You can bring it to their attention, but I'm pretty sure they know about it by now.
