The 5 Whys is a method of asking 'why' five times to get to the root cause of a problem. First state the problem:
The machine broke down:
why did it break down?
It seized up for lack of oil
why did it run out of oil?
the maintenance person did not oil it
why didn't the maintenance person oil it?
the oil filler is impossible to access
why is the oil filler impossible to access?
The machine guard conceals it
In this case it has taken four 'whys' to get to the root cause of the problem. The solution might be to cut a hole in the guard, or extend the oil filler tube.
The 5 Whys is used in Lean Production and Six Sigma.