The Great Filter is whatever has prevented life so far from developing to the point where it would colonize the galaxy.
Humanity seems to have a bright future, i.e., a non-trivial chance of expanding to fill the universe with lasting life. But the fact that space near us seems dead now tells us that any given piece of dead matter faces an astronomically low chance of begating such a future. There thus exists a great filter between death and expanding lasting life, and humanity faces the ominous question: how far along this filter are we?
-- Robin Hanson, The Great Filter - Are We Almost Past It?