Permanent resident (immigrant) is authorized to live and work in the United States.

In order to become a permanent resident of US,

1. USCIS must approve an immigrant petition for you. It could be family based, employment based or by several other means.
   
   
2. Immigrant visa number, through the Department of State, must be immediately available to you, even if you are in the United States.
   
   
3. Once an immigrant visa number is available to you, you can apply for Adjustment of Status if you are in US, or you can apply for Consular Processing for the consulate abroad.
   
   

U.S. law limits the number of immigrant visa numbers that are available every year. This means that even if the USCIS approves an immigrant visa petition for you, you may not get an immigrant visa number immediately. In some cases, several years could pass between the time USCIS approves your immigrant visa petition and the State Department gives you an immigrant visa number. In addition, U.S. law also limits the number of immigrant visas available by country. This means you may have to wait longer if you come from a country with a high demand for U.S. immigrant visas.

For immediate relatives of US citizens, there is no annual numerical limit. Therefore, immigrant visa numbers are always available to them. For other relatives and for employment based categories, there is an annual numerical limit.

For limited categories, availability of immigrant visa number depends upon the priority date.

Priority date is your place in line for getting the green card. Your priority date is based on your country of birth and preference category (EB2, EB3 etc.). The priority dates are the Department's means of establishing a waiting list for U.S. permanent residency in those Categories where the number of applicants qualified for permanent residence exceed the annual limit specified by Congress.

Current priority date means there is no waiting involved, and immigrant visa number is available.

If the priority date retrogates after filing the adjustment of status application or starting consular processing, that application would be archieved and would no longer be processed until the priority date becomes current again. So, it is wrong to think that once priority date is current and adjustment of status or consular processing is applied, priority date does not matter to you any more. Priority date always matters until you get your green card.

Family based
Your priority date is the date when Form I-130, Petition for Alien Relative was received by USCIS.

Employment based
If you have filed for alien labor certification, then your priority date is the date on which your application was received by the state level agency (SESA). If instead of labor certificate, you are going through the NIW/EB1 route, then your priority date is the date I-140 petition was received by USCIS.

Availability of immigrant visa number (priority dates) can be checked online or calling the state department telephone at 202-663-1541 (recorded message). The web site and the telephone system are updated around 15^th of each month.