When the universe was only 10 -35 seconds old, this theory states, it went through a period of inflation. Inflation, in a cosmological context, means a period of exponential growth of space-time. Explaining what triggered the inflationary period is difficult without launching into a course on particle physics. When water vapor condenses, energy is released. Similarly, when the universe went through a phase transition, energy was released, triggering a very brief but very intense period of expansion.
Inflation solves the Horizon Problem.
Before inflation, the age of the universe was 10 -35 seconds. Thus, there was plenty of time for photons to travel all through the currently observable universe, carrying energy from hot regions to cool regions, smoothing out the currently observable universe until it was very nearly homogeneous. Inflation solves the Flatness Problem. However, inflation would have flattened the universe by taking a small patch of the universe and making it vastly bigger. The usual analogy used to illustrate this point involves an ant sitting on a balloon.