As a Safety Manager, in a paragraph or two, can you explain to a new employee, the difference between doing a Safety Hazard Risk Assessment, and an Event Risk Classification?

In a paragraph or two, can you describe the difference between a $$Safety Barrier$$ and the $$Hierarchy of Controls$$ in mitigating aviation safety incidents?

An airside vehicle enters the active runway at Billy Bishop Airport. You are the SMS Manager and you receive the Occurrence Report. There is no injury or damage to any vehicles or aircraft. Severity $=$ None $(1).$ How will you define the $$Probability$$ on your risk matrix?