The United States is sometimes referred to as a “melting pot” because of the way people have immigrated to America. For the longest time, everyone saw the USA as a place where they could have a fresh start and make their situation better. For the longest time, this was true.
In coming here, people would bring their cultures and languages. Instead of insisting everyone adapt to a new culture, the USA decided that they would allow the new cultures to exist within the country and add to their existing American culture.
The idea was that America was somewhere where people of all backgrounds could live together and share their cultures. It didn’t work out quite like that, but that was the philosophy behind the melting pot mentality.