Josh Centers, writing for TidBits:
The Apple Worldwide Developer Relations Intermediate Certificate is required for all apps in the Mac App Store, including OS X installers. When used to sign an app, the certificate enables OS X to confirm that the app has not been corrupted or modified by an attacker. This certificate expired on 14 February 2016, causing error dialogs and preventing some apps from launching. Most apps affected have already been updated with the new certificate. But if you downloaded an OS X installer in case of trouble, you may be in for a surprise the next time you try to use it.