The Great Filter is the idea that, in the development of life from the earliest stages of abiogenesis to reaching the highest levels of development on the Kardashev scale, there is a barrier to development that makes detectable extraterrestrial life exceedingly rare. The Great Filter is one possible resolution of the Fermi paradox.
The Fermi paradox is the discrepancy between the lack of conclusive evidence of advanced extraterrestrial life and the apparently high likelihood of its existence. As a 2015 article put it, "If life is so easy, someone from somewhere must have come calling by now."
Personally I think it's photosynthesis. Life itself developed and spread but photosynthesis started an inevitable chain of ever-greater and more-efficient life. I think a random chain of mutations that turns carbon-based proto-life into something that can harvest light energy is wildly unlikely, even after the wildly unlikely event of life beginning in the first place.
"Photosynthesis" is a positive development for life. The great filter must be a negative development: it's a filter or a barrier that keeps life from achieving long term extra terrestrial survival.
So "climate change" would be an answer. Or "fuel depletion" (to which photosynthesis may be a solution). But the filter is the mechanism by which life forms are prevented from progressing.
I was suggesting that photosynthesis is a very unlikely mutation to occur and thus its unlikeliness means most life, if it emerges, won't progress to that stage.
The filter doesn't have to be ahead of us, it could be some stage of development that we've already passed. Like photosynthesis, or the development of consciousness. If, out of all life that develops, only a tiny fraction ever develops photosynthesis, the universe would be largely devoid of any life that we can presently detect. Despite us being the lucky lifeform that did develop photosynthesis in our past.
The failure to develop photosynthesis is the filter. I don't know how you're not getting this. No photosynthesis, no complex life, no sentience, no interstellar civilization.