Young, pampered seedlings that were grown either indoors or in a greenhouse will need a period to adjust and acclimate to outdoor conditions, prior to planting in the garden. This transition period is called “hardening off”. Hardening off gradually exposes the tender plants to wind, sun and rain and toughens them up by thickening the cuticle on the leaves so that the leaves lose less water. Hardening off times depend on the type of plants you are growing and the temperature and temperature fluctuations. There are 3 approaches to hardening off: gradually longer periods of time outdoors, a place in a cold frame and withholding water. Continue reading of hardening off in the source.
SOURCE:http://www.gardenoholic.com/how-to-harden-off-plants-outdoors/