What does ## mean for the C(C++) preprocessor?

nothing too fancy: ## tells the preprocessor to concatenate the left and right sides

see http://en.wikipedia.org/wiki/C_preprocessor#Token_concatenation

because ## is a token concatenation operator for the c preprocessor.

Or maybe I don't understand the question.

## is Token Pasting Operator

The double-number-sign or "token-pasting" operator (##), which is sometimes called the "merging" operator, is used in both object-like and function-like macros. It permits separate tokens to be joined into a single token and therefore cannot be the first or last token in the macro definition.

If a formal parameter in a macro definition is preceded or followed by the token-pasting operator, the formal parameter is immediately replaced by the unexpanded actual argument. Macro expansion is not performed on the argument prior to replacement.