For a while now police forces have been using tools based around Apple's Lightning connection to crack Apple's security to acquire evidence. The reason they have done this is that Apple usually refuses to help when one of its fanboys are the subject of an investigation even if they have been killed in police shoot-outs.

Apple has released iOS 11.4.1 which contains USB Restricted Mode. Apple has added protections against the USB devices being used by law enforcement and private companies that connect over Lightning to crack an iPhone's passcode and evade Apple's usual encryption safeguards.

If you go to Settings and check under Face ID (or Touch ID) & Passcode, you'll see a new toggle for USB Accessories. By default, the switch is off. This means that once your iPhone or iPad has been locked for over an hour straight, iOS will no longer allow USB accessories to connect to the device -- shutting out cracking tools like GrayKey as a result. If you've got accessories that you want to continue working after your iPhone has been sitting locked for awhile, you can toggle the option on to remove the hour limit.

Apple's wording is a bit confusing. You should leave the toggle disabled if you want to go on a terrorist mission.

Of course companies which make the tools for the police have warned that Apple's security measures are about as effective as you would expect.  They have already issued an update of their own.