They are Plant Hardiness Zones.
The USDA developed a map of the US with plant hardiness zones to help gardeners determine several important facts:
First planting dates.
First frost dates.
Which plants are most likely to thrive at a given location.
The average annual minimum winter temperature.
Each hardiness zone is a geographic area that includes a specific range of climate conditions that are relevant to plant growth and survival. The USDA determined the zone boundaries based on the average annual minimum winter temperature, divided into 10-degree Fahrenheit zones.
We are in Zone 6 here in Niagara County, and the USDA map shows that our county has further been divided into 6A and 6B. North Tonawanda is in 6A.
We can use this information to determine if a plant we want in our garden will survive the winter or not. Sometimes a plant will tolerate zone 6A if it is planted in an area protected from winter winds and if the soil has good drainage.
Local garden stores take this information into consideration when they choose which plants to offer for sale; however, a savvy shopper will look for this information on the plant tag.
Some plants that we consider annuals are actually perennials in southern locations. A few examples are rosemary and lantana. These plants have woody stems and can be used as shrubs much further south. Likewise, plants that thrive here due to our cooler temperatures will suffer in the heat further south, such as peony and lilac.
These links are references for this information as well as more in-depth explanations: