A plant's ability to survive winter temperatures and conditions (freezing, temperature swings, dormancy) in a given climate zone without dying.
Winter hardiness determines whether plants survive your winters. Hardiness zones define minimum winter temperatures; plants rated for your zone survive. Tender plants (tropical origin) die in freezing zones. Hardy plants survive your winters dormant. Understanding your zone and plant hardiness prevents buying plants that will die.
Choosing winter-hardy plants prevents dead plants and wasted money. Understanding hardiness zones helps you make smart plant choices.