The photon-baryon fluid stops oscillating at recombination.
The modes that freeze in at extrema of their oscillation have enhanced temperature fluctuations across the distance spanned by their wavelength:
After recombination, the universe becomes transparent, depicted here as going dark. This process is very rapid so the CMB photons don't rattle around (random walk) very much before starting to stream in straight lines unimpeded by the matter.
We'll see later that that little random walk that the photon executes at recombination is important in damping out the acoustic peaks.
