surface of last scattering
On a foggy morning the fog seems to have a 'wall' a certain distance away — not a real wall, but the depth beyond which the haze blocks your view. Everyone standing in the fog sees their own wall, at the same distance, in every direction. The surface of last scattering is the cosmic version of that wall: the most distant surface from which the light of the cosmic microwave background reaches us, the place where that ancient light scattered off matter for the very last time before flying freely to our telescopes.
When the universe became transparent at recombination, about 380,000 years after the Big Bang, the photons that had been bouncing endlessly off free electrons took one final scatter and then set off in straight lines. The light we collect today as the cosmic microwave background comes from exactly the spots that were the right distance away for their last scatter to reach us right now. Those spots form a vast sphere centered on us — not because we are special, but because light from that era arriving today must have started from one particular distance. Any observer anywhere sees their own surface of last scattering, a shell at the same look-back distance surrounding them.
It is important to be clear about what this surface is and is not. It is not a physical wall or the edge of the universe — the matter that emitted that light is still out there, long since formed into galaxies, and an alien there would see us as part of their own last-scattering surface. Nor is it perfectly thin: recombination took some tens of thousands of years, so the 'surface' is really a fuzzy shell a little thick. But it is the oldest thing we can ever see with light, our literal horizon for electromagnetic observation. To peer earlier than the surface of last scattering, we would need other messengers entirely, such as relic neutrinos or primordial gravitational waves.
Whichever way you point a microwave telescope, you are looking back to the same age — about 380,000 years after the Big Bang — and to matter now lying roughly 46 billion light-years away after all the expansion since. It is the same trick as fog: the 'wall' is always the same distance off, in every direction, because it is set by how far light from that moment can have traveled to reach you now.
The surface of last scattering is our light horizon — like the apparent wall of a fog.
It is not the edge of the universe or a real wall — it is just how far back light lets us see. The matter that emitted the CMB has long since become galaxies, and observers elsewhere have their own last-scattering surfaces.