A Priority Date is the date when the USCIS approves the establishment of an immigration application. For family-based immigrant visas, a priority date is the date the petition was filed with USCIS. It is listed on the receipt notice.
For visa categories that fall into the quota system, the United States Department of State publishes a monthly Visa Bulletin which lists the current processing cut-off dates.
You can't file an adjustment of status petition until your priority date becomes current.