Answer to Question 1
Res judicata is Latin for the thing is decided, which means that the final judgment of a competent court is conclusive. It prevents a plaintiff from suing on a claim that has already been decided, and it prevents a defendant from raising any new defense to defeat enforcement of an earlier judgment.
Answer to Question 2
Administrative law includes regulations created by administrative agencies of government. Regulations interpret how a law is to be enforced, and are generally much more detailed than the law on which they are based.