"Inflation" is the extremely fast expansion -- exponential -- of the Universe that occurred very early on -- at about 10-35 seconds after the Big Bang.
Yes, it does seem absurd to talk about such times. But, amazingly, today's particle physics does in fact predict such an expansion, and when it's probed deep it predicts quite precise values for some very subtle and esoteric parameters, such as the "spread" of values of the fluctuations from the norm of the cosmic microwave background.
In essence, the Universe is like a big old fire, now extinguished, whose heat has all rushed out to the extreme edges of what we can detect. We can look at this heat and determine that its temperature is about 3 Kelvins: -454°F.
But, like any piece of heat, the remnants of this fire are not monothilic, but has variations in it, fluctuations, just like if you look at an ember in a firepit and see it glow in one basic color but with some variations -- a bit hotter, then a bit cooler, always in flux.
These variations have been measured by satellites already launched, and they have found that the variations are quite small. But NOT zero. They're only about 10 parts per million --a very tiny part of the average temperature -- like looking at a big group of humans and detecting variations in height of 20 microns or so. In othr words, very small.
But, amazingly, today's satellites have measured these fluctuations, and today's high-energy particle theories have predicted it. And they are in surprising agreement. People, like George Smoot of LBL, have won the Nobel Prize for precisely this. It is an immense achievement, both intellectually and practically.
But thereare still more details to be rung out of the cosmic microwave background, and this is what the Planck mission will do. This will enable theorists to distinguish among competing theories of "inflation."
It is, when you think about it, quite amazing. Measurements of extremely small features of the universe -- glowing embers in a huge fire -- will enable scientists to determine which of their cosmological models make sense. Cosmology has truly been one of the greatest boones to modern physics.