It means x aliases a Fred object, but x can't be used to change that
Fred object.
Read it right-to-left: "x is a reference to a constant Fred."
For example, if class Fred has a const member
function called inspect(), saying x.inspect() is OK. But
if class Fred has a non-const member function
called mutate(), saying x.mutate() is an error (the error is
caught by the compiler; no run-time tests are done, which means const doesn't
slow your program down).