If you see the above error while trying to perform a Software Update via the Mac App Store, (an error that was pointed out by readers of another Mac-oriented website), then you need not worry too much.
The problem is all Apple's fault: In that site's very own words: "Apple Forgets to Renew SSL Certificate, Breaking OS X Software Update."
The site goes on to mention that "swscan.apple.com is one of several servers that Software Update uses to communicate with Apple. The apparent problem is that the security certificate for the server appears to have expired. The certificate was originally valid from May 22, 2012 to May 24, 2014."
This will likely be fixed very soon; I would guess 24-36 hours at the longest. But if you failed to perform the recent software updates (such as iTunes 11.2 and OS X Mavericks 10.9.3) on a timely basis and must now perform them immediately, you can always download said updates manually from Apple's support site, as pointed out by the article on the aforementioned site.
UPDATE: The issue appears to be fixed, according to the website which initially reported the situation.