Sources: background card; LLVM documentation
Bitcode is the dense binary serialization of LLVM IR. It’s used to store compiled modules on disk or embed them in object files, to ship IR between separate compilation and link-time optimization (LTO) steps, and as the distribution format for things like precompiled libraries that still want to be optimized at final link time.
Bitcode is a serialization format, not a different representation — it round-trips to the same in-memory IR. It is relevant here mainly as one of the possible export targets once a graph database of code has assembled a function’s IR: the graph is the source of truth, and bitcode (or textual .ll) is what gets emitted to hand off to the rest of the LLVM toolchain.