Celtic,
The date your Labor certification application was received by the Dept. of Labor, determines your priority date. Once your labor certification is approved, you then apply for your I-140 and I-485s. You can file them independently of each other or file them concurrently. Your I-485 application requires FBI fingerprint checks and a name check (which so far has been taking up a lot of time for many people). Once both are completed, if your I-140 is approved and your priority date is current, you will be considered for being allocated a permanent residency visa number so that your green card is issued.
With a far larger number of labor certification applications and a smaller number of available permanent residency visa numbers for each year, certain categories of I-485 applicants have to be retrogressed (which makes sense - they cannot issue green cards to all labor certified applicants). Which means applicants have to wait in queues for certain categories (EB2, EB3 etc.) or from certain countries based on their priority dates. Some countries like China, India & Mexico with a large number of applicants are more retrogressed than other countries (who are lumped together as 'Rest of the World'). You can look up current visa processing dates and whether specific category/country dates are current or not from
here.
You cannot apply for your I-485 until your priority date is current. Even after you have applied, and your finger printing/background checks are cleared and even if your I-140 is approved, your case cannot be adjudacated (finalised) unless your priority date remains current. It keeps moving up and down based on USCIS's prediction of availability of visa numbers. So even if everything else is ready, you will have to wait until your PD becomes current to actually be considered for a green card.
Ofcourse there are exceptions to all that I have said. But this is just to give you an idea of how your priority date's status impacts your green card process. One recent positive development has been that name checks past 180 days after I-485 reciept dates will no longer hold up adjudication of the application if everything else is ready and priority dates are current.
I am sure other's can add to what I have said or correct me if I'm wrong somewhere. Good luck
.
Jawad