What was the National Origins Quota Act?

I assume that you are asking about the law passed in 1924
that is commonly known either as the Immigration Restriction Act of 1924 or the
Johnson-Reed Act.  This was a law that was meant to reduce the amount of immigration to
the US, particularly from "undesireable" areas of the
world.

In the time from about 1880 to about 1920, there
were waves of immigrants coming to the US from countries in Southern and Eastern
Europe.  These immigrants were seen by many Americans as less desireable than the
Western and Northern Europeans who had made up most of the immigrants who came before
1880.  The 1924 was meant to reduce the flow of the
"undesireables."

To this end, the law set quotas based on
immigrants' nation of origin.  It pegged the number of immigrants from any given country
to 2% of the number of people from that country who had been in the US in 1890.  Thus,
it privileged Western and Northern European countries and limited the numbers from
Southern and Eastern Europe.