In the USCIS Immigrant visa petition application process, the priority date is the date the petition was filed. If the alien relative has a priority date on or before the date listed in the visa bulletin, then he or she is currently eligible for a visa.
Source: U.S. Citizenship and Immigration Services
The priority date decides a person's turn to apply for an immigrant visa. In family immigration the priority date is the date when the petition was filed at a Department of Homeland Security (DHS), U.S. Citizenship and Immigration Services office or submitted to an Embassy or Consulate abroad. In employment immigration the priority date may be the date the labor certification application was received by the Department of Labor (DOL).
Source: U.S. Department of State.