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.
National Origins Act of 1924 was to restrict the flow of newcomers from Southern and Eastern Europe. It established immigrant quotas that discriminated against Southern and Eastern Europe.