What does the flashing red frame around the screen mean?

It means an attack. Someone is marching on your base or a plot you occupy, or possibly a resource plot if you are currently gathering from it. Check the battles to see what is happening.

If there is a zombie siege event on then it could also be that the siege has started and the zombies are attacking you. This is not a problem as they do not take your resources or cause any wounded. But it's a good idea to bring in any troops that are out to help you do better in the siege score.