Nativist are a group of people who are against immigrants.
Nativism has been a part of American society since the middle of the 19th century. This belief is rooted in fear that immigrants will take the jobs of individuals who are native (aka born) in the United States. This is due to the fact that many immigrant workers will work in at jobs (in factories, on farms,etc.) for a cheaper wage than American workers. At a time where there was no set minimum wage, this was a legitimate fear for American citizens.
Along with this, many nativists feared that new immigrants would change American cultures/beliefs.