It is the universal law that will comply with all people that can rationally think and make a morally correct decision that is not based upon their own desires. To not cause harm or inflict damage upon another agent ( another person).
The Categorical Imperative deals with universalizability and states that an act is immoral if it cannot be made into a rule for all humankind to follow. For example, if I say "I will never keep my promises", this would be considered to be immoral because it cannot be applied to all people, for if it were, no-one would keep their promises and promises would lose all meaning.