What does dogen's genjo-koan mean?
Dogen is pointing to emptiness. When the self drops, you realise you are one with the ten thousand things, and there is no more I and them. Enlightenment, path, etc are no more than concepts.

The Heart Sutra:


  "Shariputra, form does not differ from emptiness, emptiness does not
  differ from form. That which is form is emptiness, that which is
  emptiness form. The same is true of feelings, perceptions, impulses,
  consciousness. "


And:


  "Therefore, in emptiness no form, no feelings, perceptions, impulses,
  consciousness. No eyes, no ears, no nose, no tongue, no body, no mind;
  no color, no sound, no smell, no taste, no touch, no object of mind;
  no realm of eyes and so forth until no realm of mind consciousness. No
  ignorance and also no extinction of it, and so forth until no old age
  and death and also no extinction of them. No suffering, no
  origination, no stopping, no path, no cognition, also no attainment
  with nothing to attain. "


http://www.kwanumzen.org/wp-content/uploads/Heart-Sutra-in-English-text1.pdf