The Melting Pot, when describing the population of the USA, refers to the way immigrants integrated the existing Anglo-Saxon culture.
Although there are some exceptions to-day, all foreign cultures were melted into the same “pot” during the very active immigration period of the 19th and early 20th century in the USA. That is all new citizens gave up their previous citizenship, used English only, gave up their traditions and most of their cultural traits, adopted new ways and developed a strong American identity.
Some other countries integrated their immigrants differently. In Canada for example we used multi-culturalism as an approach. Immigrants keep dual citizenship, keep their language but learn English or French to be used outside the home. The Canadian identity is not as strong and there are some friction areas between cultures.