Full Title or Meme
The term National Security basically means the governments are engaged in a variety of activities that they don't want to leak to enemies, either foreign or domestic.
The context is any activity of a nation that would be damaging if released.
Governments seem to feel that any embarrassing data should be protected by National Security. This attitude leads to oppressive secret policy.
The first thing to consider is that if it needs to be secret, then the government shouldn't be doing it.