If you start your plants from seeds, one of the very last steps you will need to do is prepare the plants for the rest of their life outdoors.  Plants develop a coating on their leaves that will protect them from the sun’s intense rays all day long.  Ironically, it is early exposure to the sun’s rays that trigger that mechanism. 

Since your new starts inside have not been exposed to the sun, they are not yet ready to be left out all day to that kind of exposure.  Skipping this hardening off step is akin to you going out all day for the first time after a long winter without putting on sunscreen.  The plant leaves will get burned and can die as a result. 

You should plan to harden off your plants at least one week prior to transplanting them into your garden.  This is accomplished by gradually exposing them to the sun’s rays and wind outside.  For the first day, bring your plants outside for 1 to 2 hours.  Choose an area where they will not see full sun.  For summer plants, like tomatoes and peppers, choose a day that will be above 50F.  After 1 to 2 hours, bring them back in and give them a good drink of water. 

They will probably look droopy and sad when you bring them back in.  Don’t worry, they will look better after a couple of hours recovering inside.  The next day, bring them back outside and increase the exposure time by about 1 hour.  Continue to follow this process for five days and keep increasing their exposure time about 1 additional hour each day.  After 5 days of this, your plants should be ready for transplanting and they will have the best chance of surviving any initial transplant shock.