The Great Filter is a proposed explanation for the Fermi Paradox. The development of intelligent life requires many steps, such as the emergence of single-celled life and the transition from unicellular to multicellular life forms. Since we have not observed intelligent life beyond our planet, there seems to be a developmental step that is so difficult and unlikely that it "filters out" nearly all civilizations before they can reach a space-faring stage. Robin Hanson coined the term in his 1998 essay The Great Filter - Are We Almost Past It?.
The Great Filter might be a step in our evolutionary past, in which case our civilization has already passed it. But the hard step might also be ahead of us: surviving the creation of nuclear bombs, AGI, biotechnology, nanotechnology or an asteroid impact 1. In that case, we should be worried, as the Great Filter seems to have been successful in stopping the development of every other civilization so far. Estimating the location of the Great Filter is thus important for helping estimate the magnitude of existential risk. Many efforts have been made in that direction, but much remains uncertain.
Traces of life on other planets are evidence for a later Great Filter2. If we were to find that complex life had evolved independently both on Earth and some other planet, it would suggest that getting to such a developmental stage was relatively easy. Thus the Great Filter would have to be at a later stage....