melting pot如何理解?

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; the melting-together metaphor was in use by the 1780s.
不知道你说的是不是这个...

Melting Pot：在一个地区外来的每个不同民族背景的人带来了不同的文化,最后融合形成了新的文化.
Salad Bowl：每个不同民族背景的人带来了不同的文化,但是没有互相融合,还是保有独特的文化,而这个地区的文化就是这些文化的总和.参考：http://cc.kzoo.edu/~k03hk01/melting_pot_or_salad_bowl.html
Ellis Island：坐落在纽约自由女神像旁,开始运作于1892年,主要掌管欧洲移民的移民站

Salad bowl is more proper.
The salad bowl concept suggests that the integration of the many different cultures of United States residents combine like a salad,as opposed to the more prolific notion of a cultural melting pot.In Canada this concept is more commonly known as the cultural mosaic.In the salad bowl model,various American cultures are juxtaposed — like salad ingredients — but do not merge together into a single homogeneous culture.Each culture keeps its own distinct qualities.This idea proposes a society of many individual,"pure" cultures,and the term has become more politically correct than melting pot,since the latter suggests that ethnic groups may be unable to preserve their cultures.The salad bowl has been accused of being a communitarianist model,while supporters of it speak of multiculturalism.

First,the "melting pot" and "salad" stands for the mixture of different things.
US is regarded as a "melting pot" and a "salad" is because it is a country made up of many different people:13% are black,12% are Hispanic,4% are Asian and the rest are White Americans.
As the same,the culture of US is also mingled from different culture of different people in it.
Because of these,it is really like a mixed country.