The Fermi Paradox is the illogical situation of why, with over 1 quadrillion stars in the observable universe, there has been no observed extraterrestrial signs of civilization.
The Great Filter Theory is the proposed solution to the Fermi Paradox. It proposes that there is a series of barriers that prevent life from forming and evolving into civilization, and then additional barriers exist to that civilization surviving and becoming interstellar (inhabiting more than 1 solar system). These filters can be but are not limited to: too much radiation from the main star, asteroid impact, nuclear war, climate change, or something still yet to be seen.