What does “harden off” mean?

Plants or seedlings that have been growing protected in sunny windows need to acclimate slowly to the sun’s full strength and whirling winds of outdoors.  About 1 week before planting in your garden, acclimate them to the outdoors first by leaving them in a sheltered spot outside for a couple of hours. Choose a shady spot out of the wind; then gradually extend the outdoor time by increasing the time over several days, ie. 2 hours, then 4 hours the next day, then 6 hours, until your plants fully adjust or “harden off”. Once they have spent 18 hours outside, they should be ready to spend the night.  Then, you can plant them in their ultimate ideal location!