Did you bump into this term yet and wondered what it means? It's actually not complicated. When consider to plant a palm tree in your garden it is useful to know wheather it's going to be happy with the climate conditions of your garden.
I took this definition from Wikipedia:
"A hardiness zone is a geographically defined area in which a specific category of plant life is capable of growing, as defined by climatic conditions, including its ability to withstand the minimum temperatures of the zone."
For example if a palm tree is categorized as hardy to zone 10 means that it can only withstand minimum temperature of -1°C. Check out the following hardines zone maps to learn which zone your garden is situated!
Hardiness zone maps
I only attached usda zone maps of Europe, but same kind of maps can be found for the whole world. Keep having fun!