It means the C standard does not specify what the program does. Other documents may still specify what the program does. And the program definitely still does something, whether specified or not.
No. It most definitely does not mean this. Go read the series this is part of: https://blog.llvm.org/2011/05/what-every-c-programmer-should...
It is absolutely critical that people programming in C understand what real compilers in the real world do.