Hardening plants is a process of acclimating plants from indoor to outdoor environment.
This is a gradual exposure to the outdoor. This is done when plants have been grown under grow lights or been under low sun exposure. Sudden exposure to sun can kill plants.
If plants are grown under grow lights, they have no uv exposure from the sun. And when exposed, this stresses the plants and the plants die. The photo below is my squash, seeing sunlight for the first time
To harden off, you will need to put the plant outdoors with full sun for 15 minutes on the first day. The second day, you will want to increase the time longer, 30 minutes, then 1 hour, 2 hours, 3 hours, until your plant reached the full sun exposure outdoors. Then you’re ready to fully leave them outdoors.