OBS uses a built in version of LuaJit. When OBS sets up the JIT environment, it creates an object called obslua (or if your creating a python script obspython) which contains various predefined methods that you can use to hook into the OBS internals.
All obs = obslua does is assign obslua to a variable called obs, it just saves 3 letters worth of typing .
If you are interested in making script plugins for OBS, you can check out the docs.