Depending on what `a` is, it certainly could mean something. Those are two expressions, where `f` by itself does nothing and, if `a` is a function, it will be applied to `b`. If `a` is a macro it will be transformed, and then depends on what it will be transformed into. If `a` is neither, then it's an error.
If it's inside a list already, yes. But if it's just an expression all by itself, it's not even well-formed in LISP, since it's not a list.