The national origins quota is part of the United States Immigration Act of 1924. It limited the number of immigrants allowed entry into the US based on their country of origin, setting the allowed number of visas at two percent of any given ethnic group's representation in the 1890 census.
The quota system worked to emphasize immigration from Western Europe over all other regions. At the time, it also effectively closed the US to any further Asian immigration. Considered a betrayal of an earlier agreement, this particularly taxed diplomatic relations with Japan. The restrictions put in place were revised in 1952.