The white replacement conspiracy theory holds that Jews, recognizing the cultural and biological superiority of the white/European race (which they are separate from), are secretly manipulating world events and social structures in order to reduce whites to a minority even in historically majority-white nations and regions. They're doing this in two ways - firstly by inventing and promoting technologies and social attitudes (like liberalism, soy foods, interracial marriage, etc) that directly reduce the white birth rate, and secondly by causing or instigating things like refugee migrations, migrant caravans, etc to flood white countries with non-white people who reproduce at higher rates.
It is one of the oldest "Jewish conspiracy" theories and is the specific brand of antisemitism that motivated Adolf Hitler and Henry Ford.