Hardening off is a method of introducing plants from the warm environment of the greenhouse to outdoor conditions.
Failing to do this can result in plants being shocked so severely, it may check the growth or even kill them.
What is required is a process that gradually allows the plant/s to adapt to outdoor conditions, a half way house if you like.
The most common way of doing this is to use a coldframe as an intermediary location.
A few days after pricking out, and providing the plants have settled into their growing medium, move them into a closed cold frame.
Gradually increase the ventilation by opening the lid of the coldframe for a few hours in the warmth of the afternoon.
After a few days if conditions allow, leave the frame open all through the day, just closing it up at night.
By the end of a week to ten days you should be fine to leave the frame open day and night, just sheltering the plants from the worst of the cold.
Various stages of Hardening Off
After two weeks plants are usually sufficiently hardened off to be planted out.