FBtoC is a replacement for the old FB Compiler (FB Compiler.app). Translating the code to C allows Apple's developer tools to create a universal binary. The diagrams below illustrate the process:

FB Compiler

FB source code ------> legacy app (PPC-only, CFM format, resource fork...)

FBtoC

FB source code ------> C source code ------> OS X app package with Carbon universal binary in Mach-O format