what is anti-terrorism law?

Answer 1

A law that says I can call you a terrorist without any supporting evidence and then have you thrown in a prison outside the judicial system where I can treat you anyway I want until I have manufactured the evidence needed to prove you are a terrorist.

Answer 2

Terrorism is usually defined in law as attacks against civilian populations for the purpose of causing civil unrest.

Anti-terrorism laws are those that criminalize such conduct, or allow for investigative or other secondary provisions to prevent such activity.