The Assert action is a simple new action in Keyboard Maestro 8. It is basically just "if not condition then fail", but this can be a very useful tool for ensuring that your macro has received the right data.
For example, if your macro is expecting you to have copied a URL:
This ensures the macro stops immediately if it is given the incorrect data.
Asserts are a very powerful tool for ensuring correctness, helping you quickly track down and resolve a problem when the macro has gotten the wrong inputs.