A melting pot is a metaphor for a society where many different types of people blend together as one. America is often called a melting pot.
Some countries are made of people who are almost all the same in terms of race, religion, and culture. Then there are places like the United States, where there are many different types of people. A place like that is a melting pot, because the people — despite their differences — manage to coexist and live together as one nation. Anywhere diverse people are assimilated could be called a melting pot. In a melting pot, differences become less important than unity.