Hubble's constant, approximately 70 km/s/Mpc, describes the rate of expansion of the universe, indicating how fast galaxies are moving away from each other. This expansion is linked to cosmic inflation, which suggests that the universe underwent rapid expansion shortly after the Big Bang, affecting the observable horizon. The Hubble horizon represents the maximum distance from which light has had time to reach an observer since the beginning of the universe, influenced by the rate of expansion. Understanding these concepts helps clarify how the universe's expansion impacts what we can observe. The relationship between Hubble's constant and cosmic inflation is crucial for grasping the dynamics of the universe's growth and its observable limits.