A long time ago in a galaxy far, far away... If you look up at the night sky in any direction, past all the stars and more stars and galaxies and superclusters of galaxies, you will see light that has been traveling for 13.7 billion years to reach earth. It's the oldest and most primeval light in the universe, a picture of our cosmos in its hot younger years, and it's called the "Cosmic Background Radiation" (aka CBR or CMB).
The very fact that there IS a faint glow emanating almost identically from every direction in space is supporting evidence that our familiar cosmos came from small beginnings. That's because the simplest explanation for such a uniform glow is that the whole universe was all, a very long time ago, compressed into pretty much the same place. Otherwise it's very hard to explain why the glow shouldn't be a lot brighter or hotter in some places (and thus, certain directions) than others.
One of the reasons we call this small dense stage of the universe the "beginning," is that if we roll back the clock a mere 380,000 years before the cosmic background radiation popped onto the scene, the universe would have been cozied-up with itself enough that our standard definitions of space and time no longer work. It's like if you took a newspaper, mashed and crumpled it all up into a tiny ball, and asked what order the pages were in. With the front page tangled with the sports section here and underneath the classifieds there, that question just doesn't make any sense.
But once the universe became a bit more stretched out and reasonably sized, talking about time makes sense again. And for the next 380,000 years, with everything jammed close together, the universe would have been very hot, very dense, and surprisingly smooth: it just hadn't had time to get stirred up yet. The evidence also suggests that the universe was growing in size incredibly quickly – a bigger and bigger bang! For a while, it would have been so hot that electrons didn't settle down as parts of atoms or molecules but instead roamed freely, scattering off of protons and other electrons in a kind of cosmic soup. That red-hot soup would have had lots of light bouncing around, getting absorbed and reemitted by electrons and protons in a kind of quantum hall of mirrors, not unlike the interior of the sun...
However, as the universe expanded, there was less energy to be had in any one place. And when it had cooled to just below the temperature of the sun, pairs of electrons and protons no longer had the energy to resist each other, so they fell into the electromagnetic embrace we call the hydrogen atom. As the temperature continued to fall, the electrons became more and more enamored with their new proton love interests and soon began to ignore the light bouncing around them altogether. So they stopped acting like mirrors, the universe became transparent and suddenly, like a feather thrown to the wind, light was free to travel lonely and unnoticed through the cosmos. Until finally, after billions of years, it arrives here on earth.
Of course, this light has been stretched so much by the thirteen-billion-year expansion of space that, like a record slowing down, its frequency and color have shifted from the original sunlight-white all the way to cool microwaves by the time it reaches us. Thus, it's often called the "cosmic Microwave background radiation, or CMB"… but give the universe a few billion more years to expand, and the background radiation will have shifted beyond microwaves and far into the radio spectrum, just like the CBR has already been shifted from the sun-like visible wavelengths at which it was emitted to the microwaves we see today. And just as we can tell the temperature of a red or white-hot iron from its glow, so too does this light tell us the temperature of empty space: currently around 2.725 Kelvin, or minus 270 degrees Celsius.
However, the universe isn't exactly 2.725 K everywhere… if we look closely, there are small and seemingly random but noticeable bumps all over the place, kind of like milk that's starting to curdle. Our best understanding is that these cosmic curds formed as quantum fluctuations in the temperature and density of the otherwise evenly-mixed infant-formula universe, and then began to coagulate as the universe cooled and expanded.
Some of this clumping resulted in small variations in the temperature of the cosmic background radiation, and these variations are what we see in the images taken by WMAP and Planck satellites and what we've represented as mountains and oceans in the Big Bang Registry "Qualitative Geography of the Cosmic Background Radiation" map.
It's hard to overstate just how small, or un-bumpy, these fluctuations of temperature and density were to begin with - the hot or cold spots were hotter or colder than their surroundings by about the same amount that the flowing tides affect the radius of the earth or that a bacteria makes a beach ball bigger.
Later, of course, the small chunky curds of primordial soup attracted gravitationally, eventually congealing, growing, and accreting to form all of the massive/immense structures in the universe that we see today, like stars, galaxies, and superclusters of galaxies.
So the light reaching us from the cosmic background radiation is a literally snapshot of the starting point from which the rest of the universe sprang. Or quite simply, the first baby picture ever taken.
An interesting side note: if our solar system had formed a few billion years earlier, light traveling across the universe wouldn't have had as much time to get stretched and we would be seeing the CBR in infrared rather than microwaves. That, in turn, means we probably would have taken a lot longer to discover it, because infrared, light is largely blocked by water and other molecules in the earth's atmosphere – precisely the same mechanism that causes heat to be trapped a la the greenhouse effect. Except that for infrared light coming from outside, our atmosphere would have acted like a reflector, keeping the CBR out!