AKAIK It refers to demographic trends in the Western world where white ppl have lower fertility rates than other groups.
The actual term is "great replacement", and it's a conspiracy theory regarding an alleged systematic replacement of native Europeans (i.e. "whites"). The Christchurch shooter was a big fan, apparently.
It's a way to relabel normal immigration by upstanding people into nefarious "white genocide".
I agree with you, but you should have expressed this in a way that didn't assume bad faith.