The US tightened its immigration policies during this time by restricting the numbers of immigrants who could come to the US. The laws also set quotas for immigrants from various countries so as to allow only the "right" kind of immigrants to come.
An example of this was the National Origins Act of 1924. This law said that only 150,000 immigrants per year could enter the US. It also said that countries would get quotas as a proportion of how many people from that country had been in the US at the 1890 census. That meant that fewer of the "new immigrants" from Southern and Eastern Europe would be allowed in because there had not been many of them in the US in 1890.