These animations were made to introduce the Integrated Sachs Wolfe effect for a talk. They depict a CMB photon passing through a gravitaitonal potential well associated with a large scale structure overdensity.
During the matter dominated era of the universe, the potential is static so there is no net impact on the photon energy.
During the dark energy dominated era, the potential is decaying, so the photon effective exits a shallower well than it fell into. This results in a net gain of photon energy.