The melting pot is a metaphor for a heterogeneous society becoming more homogeneous, the different elements “melting together” into a harmonious whole with a common culture. It is particularly used to describe the assimilation of immigrants to the United States.
Over time, Italian-Americans married Russian-Americans, Irish-Americans married French-Americans, and the next generation would be just plain American.
The Left wants a country of mutually-hostile factions, to make it easier to control by "community organizers". They thus encourage the immigration of populations least-likely to easily assimilate.