Emscripten is an LLVM to JavaScript compiler. It takes LLVM bitcode (which can be generated from C/C++ using Clang, or any other language that can be converted into LLVM bitcode) and compiles that into JavaScript, which can be run on the web (or anywhere else JavaScript can run).

Using Emscripten, you can

Compile C and C++ code into JavaScript and run that on the web

Run code in languages like Python as well, by compiling CPython from C to JavaScript and interpreting code in that on the web