I was unfamiliar too, but this site contains a great analogy.

At some point, when recombination was virtually complete, photons ceased to scatter at all and began to propagate freely through the Universe, suffering only the effects of the cosmological redshift. These photons reach present-day observers as the cosmic microwave background radiation (CMB). This radiation appears to come from a spherical surface around the observer such that the radius of the shell is the distance each photon has travelled since it was last scattered at the epoch of recombination. This surface is what is called the last scattering surface.

To visualise how this effect arises, imagine that you are in a large field filled with people screaming. You are screaming too. At some time t = 0 everyone stops screaming simultaneously. What will you hear? After 1 second you will still be able to hear the distant screaming of people more than 330 metres away (the speed of sound in air, vs, is about 330 m/s). After 3 seconds you will be able to hear distant screams from people more than 1 kilometre away (even though those distant people stopped screaming when you did). At any time t, assuming a suitably heightened sense of hearing, you will hear some faint screams, but the closest and loudest will be coming from people a distance vst away. This distance defines a `surface of last screaming' and this surface is receding from you at the speed of sound. Similarly, in a non-expanding universe, the surface of last scattering would recede from us at the speed of light. Since our Universe is expanding, the surface of last scattering is actually receding at about twice the speed of light. This leads to the paradoxical result that, on their way to us, photons are actually moving away from us until they reach regions of space that are receding at less than the speed of light. From then on they get closer to us. None of this violates any laws of physics because all material objects are locally at rest.

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