What does the clang -cc1 option do?

The Clang compiler front-end has several additional Clang specific features which are not exposed through the GCC compatibility driver interface. The -cc1 argument indicates that the compiler front-end is to be used, and not the driver. The clang -cc1 functionality implements the core compiler functionality.

So, simply speaking. If you do not give -cc1 then you can expect the "look&feel" of standard GCC. That is the vast majority of compiler flags work just like you would expect them to work with GCC. If you pass the option "-cc1" then you get the Clang compiler flag set. Thus, there might be differences to GCC.

Hope that makes it a little clearer.

The usual compiler consists of so-called compiler driver, which knows how to execute compiler itself, assembler, linker, etc. and compiler itself which just takes the source code (sometimes already preprocessed) and emit assembler/object code.

Clang implements all these components in the single binary, the difference is just the cmdline. Here clang -cc1 invokes the compiler itself with its internal/undocumented set of options, etc.