Passcodes have pretty much become the standard security measure of choice for most iPhone users. Even in the presence of more advanced biometric solutions, like Face ID, the sheer convenience and approachability of a four, six or even longer digit number, makes it the ideal fallback security measure. The way it works on iOS is simple, yet efficient - you get a total of 10 attempts to enter the code. Fail all of them and the data will get automatically wiped, for security. The number of input attempts is tracked by a hardware module, called the Secure Enclave, making it pretty impossible to actually disable the limit or circumvent it directly. As an extra any brute-force measure, each consecutive pin entry has a slightly longer processing time.
Now for the magic. The way this attack works is by attaching an external input device to the iPhone. One simulation a keyboard, to be exact. A hacker, going by the name "Hickey", figured out that instead of entering codes one by one and then waiting for a validation, you can actually generate all the combinations in a single long string of inputs, without any spaces and send it over to the phone. Apparently, iOS will still attempt to process all the numbers. The other part of the trick stems from the fact that the keyboard input takes precedence over the wipe data command. So, in effect, the Secure Enclave is still counting your failed attempts, but the actual wipe can't occur before the phone is finished processing the inputs. That means that if you iterate through all the possible combinations, you will eventually unlock and cancel out the wipe command.
Now, "eventually" is the operative word here. A four digit passcode typically takes between three and five seconds to process. That roughly equals an hour for just 100 combinations. And you do have 9999 to go through, in the worst case scenario. Things ramp up quickly with six digit codes - which is now the default length on iOS. Still, it is interesting to see that particular brute force attack has been executed successfully even on iOS 11.3.
That being said, Apple hasn't remained oblivious to such issues, since this is far from the only method for circumventing iPhone security out there. Companies, like Grayshift have actually constructed an entire business model, based on such activities. To combat this, iOS 12 has, what is know as a USB Restricted Mode. It prevents the Lightning port from being used to communicate with other devices, if the phone hasnt been unlocked for over an hour. That makes using methods, like Hickey's brute force attack a lot harder, but definitely not infeasible.