A more thorough explanation:

The National Visa Center (NVC) is a center that processes visas for people who want to come to the United States. It is run by the U.S. Department of State’s Bureau of Consular Affairs.

When someone applies for an immigrant visa, the U.S. Citizenship and Immigration Services (USCIS) approves the petition and sends it to the NVC. The NVC collects the visa application fees and documents and keeps the petition until it is ready to be reviewed by a consular officer abroad.

Depending on the visa category and the country of birth of the visa applicant, the petition may stay at the NVC for several months or even years.

John wants to come to the United States to live with his wife, who is a U.S. citizen. His wife files a petition with the USCIS, and once it is approved, the petition is sent to the NVC. The NVC collects the necessary documents and fees from John and his wife and keeps the petition until it is ready to be reviewed by a consular officer at the U.S. embassy in John's home country.

This example illustrates how the NVC serves as a middleman between the USCIS and the U.S. embassy abroad. It collects the necessary information and documents and keeps them until they are needed for the visa application process.