Hardening off
Hardening off involves acclimatising plants to outdoor temperatures so they can survive outside without being damaged by frost or cold.
Plants raised indoors in warmth are soft and likely to suffer if you suddenly expose them to the elements. But you can avoid this by gradually acclimatising them to lower temperatures for a week or two beforehand.
This can be done by standing the plants in a sheltered place outside on mild days, just for a few hours at first but slowly lengthening the time until they're eventually left outdoors all day and night. Alternatively, you can move them to a cold frame. Keep the lid closed to start with, and then admit a little air, increasing this daily until the frame is left wide open all the time, after which the plants should be ready to plant out.