What does __VA_ARGS__ in a macro mean?

It’s a variadic macro. It means you can call it with any number of arguments. The three ... is similar to the same construct used in a variadic function in C

That means you can use the macro like this

DEBUG("foo", "bar", "baz");

Or with any number of arguments.

The __VA_ARGS__ refers back again to the variable arguments in the macro itself.

#define DEBUG(...)  printString (__VA_ARGS__)
               ^                     ^
               +-----<-refers to ----+

So DEBUG("foo", "bar", "baz"); would be replaced with printString ("foo", "bar", "baz")