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.
The new setting means that after your phone is locked for one hour it’s USB port is essentially rendered useless.
There’s already a workaround for it though (kinda), where you basically keep something plugged into it when the phone is first obtained. That of course requires you to get the phone within the hour window before it locks off the USB port.
Source: Apple releases iOS 11.4.1 and blocks passcode cracking tools used by police – The Verge