The first thing that you should keep in mind about residency availability and priority dates is that for many categories you do not have to even worry about priority dates. The most common of these are individuals getting their green cards as the immediate relative of a United States Citizen. These residencies have no maximum amount but you still may have to wait due to bureaucratic inertia.
The most common examples of people getting residencies as immediate relatives are:
The husband or wife of a US citizen
Any unmarried children under the age of 21 of US citizen
The parents of US citizens who are at least 21 years or older
The widows or widowers of United States citizens. This applies in two situations.
When a citizen spouse filed a petition on their behalf before they died or If the widow or widower files a petition within two years of the citizen's death.
Individuals who have asylum also do not have to wait for a priority date. However, they can't file for their green cards until at least one year has passed since the grant of Asylum.
For all other types of residencies, with priority dates usually determine whether you will be able to get your residency right away, or whether you are on a waiting list and where you are on that list. This is due to limitations in the number of residencies available in each of the categories (known as preferences).
In general, these preferences fall into two broad categories:
Family-sponsored preferences (limited to 226,000 visas per year)
Employment-based preferences (limited to 140,000 visas per year).