Stripped of the formal wording, this guidance comes down to one idea: when you leave your vehicle you should - remove the ignition key and engage the steering lock - lock the car, even if you only leave it for a few minutes - close the windows completely - never leave… It is advice rather than law, but examiners and the courts still treat it as the expected standard of safe driving.
It belongs to the annex 6. vehicle maintenance, safety and security part of the Code, the habits a confident, considerate driver builds until they are automatic. The aim is not to memorise the sentence word for word, but to understand the hazard it protects you from, so you apply it without having to think when it counts.
If you are learning, treat this rule as one piece of a connected set rather than an isolated fact. The related rules below sit in the same section and reinforce each other, reading them together is how the annex 6. vehicle maintenance, safety and security part of the Code starts to feel like common sense rather than a list to revise.
Because this is advisory rather than legal, no one will fine you for the rule alone, but ignoring it can still count against you in a careless-driving case, and it will cost you faults on the test. Either way, the safe move is to build the habit early, while a driving instructor can correct it, rather than relearning it under test pressure. That is exactly what the practice routes and coaching in the DriveRoutes app are designed to help with, turning the rules below into the way you naturally drive.