For years the horizon problem was the most troubling glitch in the Hot Big Bang model. The problem exists because light has a finite velocity and can only have traveled a finite distance since the Big Bang. As a result of this property, at every point in the Universe there is a sphere with a radius of 14 billion light years called the visible Universe.
Observations show that the CMB is very nearly isotropic. This phenomenon is best explained if different regions of the sky could have interacted in the past and been driven towards thermal equilibrium. However, consider points A and B in the CMB on opposite sides of the sky; they are separated by almost 28 billion light years, far longer than light has been allowed to travel since decoupling. If no signal could traverse the sky then there would have been no way to establish thermal equilibrium. Similarly there would be no way to generate the thermal irregularities that are the seeds of stars and galaxies.