What does Bilbo finally communicate about himself in these concluding words? What does Gandalf mean when he says, upon hearing these words, "You are not the Hobbit you were."?

As Bilbo approaches his home he recites a poem,part of which follows:

Road go ever ever on

under cloud and under star

yet feet that wandering have gone

turn atlast to home afar

Eyes that fire and sword have seen

and horror in the halls of stone

Look at last on meadows green

And trees and hills they long have known.