LLVM is the compiler infrastructure that underlies Clang, Vuo, and many other projects. It’s a set of libraries to help you build compilers (and more). Among other things, LLVM provides a C++ API for generating LLVM Intermediate Representation (LLVM IR) code. LLVM IR is an assembly language for a hypothetical computer. LLVM IR code can be either interpreted or compiled down to native code.
So LLVM provides this C++ API for generating LLVM IR code — but it doesn’t stop there. LLVM can also generate C++ code that generates LLVM IR code. In other words, LLVM can literally write part of your compiler for you!