Plant hardiness zones are categories created by the USDA to help people understand which plants are likely to thrive in certain environments. It’s a system calculated from the coldest temperature of the year at each location, averaged over a 30-year period. Hardiness — in this context — refers to the likelihood of a plant dying from the cold.
I’m writing about this because, apparently, plant zones borders have moving upward over the past few decades. The NYTimes has a more comprehensive piece on this than I do, so I would recommend reading their page if you want to get full story; they ask a few growers in the piece how their gardening habits have changed, and it’s nice to hear from the practitioners themselves about the problem.