USA Hardiness Zones
Because the USA is so large and covers so many different climatic regions, a zone system of plant hardiness has been devised to make it easier to decide if a plant will grow in your particular plot. The zones are based mainly on the average annual minimum temperature that is found in a region as that will determine the long term survival of a plant.
When you see pants for sale, they will be marked with "Zones 4-8" for instance, this shows that the plant should be hardy if planted in those zones.
Note that the zone in a particular area can be affected by microclimate and also elevation. If you are in zone 5 for instance and live fairly high up a hill, facing largely north with wide open spaces around you, your effective zone could be 4 . Likewise if you live in a sheltered valley, facing south and surrounded by forest or even rock faces, your zone could be 6. It is even possible for the microclimate in different parts of your garden to have different zones depending on aspect, exposure and shelter.
This all means that the zones recommended for a plant are only a guide. If you're in In zone 5 for instance and a plant is recommended for 3-7, then all should be fine, but look at a zones 5-9 or 1-5 plant and local conditions start to become more relevant. There are no real hard and fast rules in gardening,
The zones are determined according to the average annual low temperature, the lowest temperature that can be expected in a year on average. You might get a year or more than one in a row where the annual temperature doesn't get as far as the long term average so allowing a more tender plant to survive than might be expected, or conversely a particularly cold year may kill plants that in other years would have survived.
More detailed map below.