Hardening off plants is a method to gradually exposing plants started indoors to outdoor growing conditions. Think of it this way- you start seeds inside and they spend the next several weeks in a nice happy spot with all of the perfect growing conditions. You give your seedlings the right lighting, moisture, temperature, etc. They are not exposed to any elements of nature such as wind, overcast days or extremely sunny days, hard rain, sudden changes in the temperature, etc. If you want to minimize transplant shock and set your transplants up for success, harden off your plants before putting them in the ground.
Hardening off is done by exposing them outdoors for a few hours a day and gradually building up to leaving your plants outside overnight. This can be done by just placing them somewhere in a semi-shady spot for a few hours a day. Increasing an hour or so each day in a spot with more direct sun exposure. Eventually you will leave them outside for a full night. This can be done anywhere from 7 to 14 days before you plan on planting your plants outside depending on the conditions in your location. The temperature should be at least 45F in the daytime before you bring plants outside to be hardened off. Wait until it is at least 45F overnight before hardening off plants overnight (mid 50s for warmer crops). Avoid extremely windy spots as they are damaging to young plants. Reduce the amount of water plants receive, but do not allow them to wilt. This will help them grow stronger as well, especially if you have different irrigation methods for outdoor growing.
Another method of hardening off is by placing them in a cold frame. A cold frame is a bottomless box that is set over plants in the garden to protect them from cold weather. They are usually built low to the ground and have a transparent roof to let in light. There are multiple ways to construct a cold frame using a variety of materials. Some cold frames are not boxes and resemble a miniature high tunnel- with the frame and some sort of fabric or plastic on top. The cold frame is going to act as a barrier against wind and help keep a warmer environment for your plants while still allowing them to experience the sunlight and ground temperature. You follow the same guidelines on not leaving your young plants outdoors when it is less then 45F overnight. You can use a portable cold frame and provide protection to your plants after transplanting them by placing the cold frame over them for a few days. Some people chose to direct sow and use their portable cold frame to provide a less harsh environment for seedlings as they grow.
Remember, the goal is to gradually acclimate your plants to survive outdoors. You will need to adjust the hours and sun exposure each day to slowly build up to the overnight stay and eventually permanently planting in the soil. Hardening off is important for your plants to adjust to a completely new living environment.