Ordinarily, rubidium atoms are opaque. But after being hit with the second laser beam, they became transparent. In that state, photons were able to pass through the atoms, albeit slowly.
The photon didn't just slow down, either. Because the the rubidium atoms were in such an excited state, only one photon at a time was able to pass through the atom. When another photon entered the cloud of atoms, the rubidium becames opaque and the photon was unable to pass through.
Turning off the control beam also rendered the rubidium atom opaque, blocking any photons from passing through. That means the apparatus is an optically controlled switch, one that works on single photons.
The effect also implies that one could build a kind of quantum transistor. A rubidium chamber emitting single photons could be placed next to another just like it. The single photons would go to the second chamber. The first ones to get there could be let through while later ones would be stopped. Turn off the first chamber for a moment and the second one "opens" again.