Priority Dates and How to Read the U.S. Visa Bulletin
What is a priority date?
According to Cornell Law School:

"The priority date is used by the Department of State to determine an individual’s place in line in the visa queue where there are a limited number of immigrant visas available in a given year. For family sponsored immigration, the priority date is the date that the petition is properly filed with U.S. Citizenship and Immigration Services (USCIS). The priority date for an immigrant petition that is based on employment is either the date the petition was filed with USCIS, or the date the labor certification application was accepted for processing by the Department of Labor.

When the priority date becomes current, the individual will be eligible to apply for an immigrant visa. The United States Department of State publishes a monthly visa bulletin which lists priority dates for different immigration categories and birth countries, allowing individuals to check their place in the visa queue."

Where do I find my priority date?
If you have filed a permanent residency petition, you can find your priority date on Form I-797, Notice of Action, for the petition filed on your behalf.

For more information, read the Murthy resource linked below.
Additional Resources:
Murthy Law Firm - Priority Dates: How Does the Visa Bulletin Work?
USCIS Visa Availability and Priority Dates
Visa Bulletin