Nativism is essentially an anti-immigrant attitude. It is the feeling that the “natives” of a particular country are in some way better than immigrants and that immigrants will be bad for the country as a whole. There have been surges in nativist attitudes each time the US has had large waves of immigration.
Broadly speaking, there have been three main waves of immigration to the US. In the 1840s there was a large wave of immigration, mainly from Ireland and Germany. Between roughly 1890 and World War I, there was another wave. This wave came mainly from Eastern and Southern Europe. Finally, there has been another wave in our own time. This wave has consisted mainly of Asians and, in particular, Hispanics.
In all cases, there have been nativist reactions. Nativists believe that “native” American people and their culture is superior and should be maintained. They worry that immigrants will bring foreign ways that will weaken the US. They believe that immigrants will not want to assimilate and become Americanized. They worry that immigrants will be more prone to criminal actions. In short, they are very suspicious of immigrants and they think that the country would be better off with fewer or no immigrants.