Gatekeeper is one of the Mac OS built-in protection against malware. On a new installation of the system is set to only allow apps from Apple, the Mac App Store and the "specified developer."
The latter means developers who have signed their program with a certificate from Apple, which makes the system can verify that the program has not been tampered with since it was signed.
Mac OS Sierra came a change in how the system handles these signed applications. If a developer's certificate has expired, the program can no longer run. Previously, only that the program was counted as a non-signed applications, but now it does not help to turn the Gatekeeper – a signed application with expired certificate crashes immediately with "code signature invalid" as an error message.
Among the programs who suffered in the weekend there 1Password, PDF Pen Pro and Soulver. To get around this, you must manually download and install an update, because the programs can not be started and run its update services.
For programs under active development, this could be seen as a speed bump and not a major problem, but it can have greater impact when older programs that no longer developed expires and can not be started regardless of whether the user is aware of the risks. Hopefully Apple can fix it so that it is possible to turn off signature verification for individual programs.