Hardening off is basically reducing stress on young plants by gradually getting them used to a harsher environment than the one they’ve been growing in. This applies to hardy plants as well as tender plants, vegetables, fruit, and flowering plants.
The easiest way to do this is to put your seedlings outside during the day and bring them in overnight for at least seven days. Then, they can be planted in their permanent growing position for the year, ideally in a slightly shady spot sheltered from excessive wind.
If you have cold frames, you can also put the seedlings into these for seven to fourteen days. Simply close the lid at night until the seedlings are ready to be planted out.