The surface of last scattering (aka last scattering surface) is the spherical shell shape around us from which the photons of the cosmic microwave background (CMB) are arriving. What we "see" as we observe the CMB is the surface of last scattering. It is the most distant thing we see, everything else we observe being in front of it.
Before recombination, the photons were constantly scattered within the dense plasma of the time (something like light within fog). Pretty much by definition, recombination indicates the time when photons began moving freely (free streaming) and photons that happened to be headed are way constitute what we are observing. Photons that happened to begin their free streaming at a point closer to us have already reached us or are off in a different direction, and photons that began their free streaming further from us have yet to reach us. The surface of last scattering constantly moves away from us, both due to time passing and due to the Hubble expansion, but in our lifetime, the increase is not overly significant.