From Yahoo Finance: “…the FBI said that if can’t require Apple to create the weakened software, it may demand access to what it described as Apple’s “crown jewels” instead. Source code is the list of programming code instructions used to create the software that runs the iPhone. The code controls everything from the background colors on the screen to the most critical security protections the phone has. Apple’s secret signature is a digital “key” required to update software on all iPhones.
If the FBI got access to those two items, the bureau, or outside programming experts it hired, could try to write the security-weakened version of iOS and install it onto Farook’s iPhone without Apple’s assistance.”