The Universe spent its first few hundred thousand years as a plasma: very hot, completely ionized, with photons interacting constantly with free electrons. At the end of this era, the Universe got cool enough for the first atoms to form and stay intact; the average photon energy was no longer high enough to ionize hydrogen. So the photons "decoupled" from matter, and were able to travel long distances without being absorbed. The temperature of the Universe at that time was around 3000K, so the photons that filled it would have had roughly the same spectrum as the filament in an incandescent lamp: lots of photons in the visible range.
Photons from the time of decoupling are still travelling through the empty spaces of the Universe, but they have been redshifted by the continuous expansion of the Universe in the billions of years since decoupling: redshifted about 1100-fold, so that they now have a spectrum which corresponds to a temperature of just 2.7K. That places their peak wavelengths in the microwave range: hence "cosmic microwave background".
When we detect photons in that spectrum, we know they've been travelling for 13 billion years, since they decoupled from matter. The actual distance they've travelled is complicated by the expansion of space while they've been travelling, so you'll see various numbers quoted for that value. We can't detect photons that come from farther away, because they would have been emitted during those first few hundred thousand years when the Universe was composed of plasma, and they would have been almost immediately absorbed by interactions with the sea of free electrons in the plasma. And we don't see cosmic microwave background photons from nearer at hand, because all the CMB photons were produced at the same time in the Universe's history, and the ones that were produced close to us have now passed us by and are on their way somewhere else; perhaps an observer billions of light years away is picking up CMB photons that were emitted in our vicinity.

Grant Hutchison