Dan Guido has a good piece on the technical aspects of what the FBI wants Apple to do:
Again in plain English, the FBI wants Apple to create a special
version of iOS that only works on the one iPhone they have
recovered. This customized version of iOS (*ahem* FBiOS) will
ignore passcode entry delays, will not erase the device after any
number of incorrect attempts, and will allow the FBI to hook up an
external device to facilitate guessing the passcode. The FBI will
send Apple the recovered iPhone so that this customized version of
iOS never physically leaves the Apple campus.
As many jailbreakers are familiar, firmware can be loaded via
Device Firmware Upgrade (DFU) Mode. Once an iPhone enters
DFU mode, it will accept a new firmware image over a USB cable.
Before any firmware image is loaded by an iPhone, the device first
checks whether the firmware has a valid signature from Apple. This
signature check is why the FBI cannot load new software onto an
iPhone on their own — the FBI does not have the secret keys that
Apple uses to sign firmware.
Guido thinks the situation would be very different if the iPhone were newer than a 5C:
At this point it is very important to mention that the recovered
iPhone is a 5C. The 5C model iPhone lacks TouchID and, therefore,
lacks the single most important security feature produced by
Apple: the Secure Enclave.
If the San Bernardino gunmen had used an iPhone with the Secure
Enclave, then there is little to nothing that Apple or the FBI
could have done to guess the passcode. However, since the iPhone
5C lacks a Secure Enclave, nearly all of the passcode protections
are implemented in software by the iOS operating system and,
therefore, replaceable by a firmware update.
★ Wednesday, 17 February 2016