You needn't lock the stable door when the steed is stolen.

Precautions taken only after the harm has happened cannot undo it. The saying criticizes a response that addresses a danger after the opportunity to prevent the loss has already passed.

It can also urge foresight: identify vulnerabilities and act while protection can still make a difference. The point is not that later action is always worthless, but that it should not be mistaken for prevention of the original damage.

Use it for an overdue safeguard after a predictable failure, loss, or breach. It is a mildly critical reminder that prevention must come first; avoid it when later measures can still limit harm, or immediately after a serious personal loss, when the figurative image may sound insensitive.