chrisaycock's answer is complete, but here's the relevant lines in the article:
The code generator is one of the most complex parts of LLVM. Its task is to "lower" the relatively high-level, target-independent LLVM IR into low-level, target-dependent "machine instructions" (MachineInstr).
The code generator is one of the most complex parts of LLVM. Its task is to "lower" the relatively high-level, target-independent LLVM IR into low-level, target-dependent "machine instructions" (MachineInstr).