A geographic classification (numbered 1-13 by USDA) based on average minimum winter temperatures, indicating which plants can survive winters in that region.
Hardiness zones divide North America into regions by winter coldness. Zone 3 is very cold (-40°F minimum); Zone 10 is mild (20°F minimum). Your zone determines which plants survive your winters. A plant hardy to Zone 5 survives Zone 5 winters but may die in Zone 3. Seed packets and plant tags list hardiness zones; choose plants for your zone.
Knowing your hardiness zone prevents buying plants that won't survive winter. It's the foundation of choosing appropriate plants.