'Hardening off' sounds callous, but it's actually not. In short, it's what one should do to avoid seedling death.
Here in western Washington our warm weather growing season is relatively short, so indoor seed starting helps us get a head start on the weather. The problem with starting seeds indoors or in a greenhouse, however, is that the plants are kinda wimpy. Transferring a seedling that has only known warm, non-windy, mild UV conditions to the great outdoors will shock it to say the least. To avoid serious transplant shock, and possibility of mortality, seedlings need to be 'hardened off'. Hardening off is a process of getting seedlings used to outdoor growing conditions by transferring them outdoors during the day, and then bringing them back in at night. One should do this for about a week before transplanting seedlings into outdoor garden beds.
If you are buying seedlings from a nursery, they should already be hardened off, so don't worry about this. For more detailed information about hardening off, visit this WSU informational page http://gardening.wsu.edu/library/vege003/vege003.htm
I finally got these cool weather seedlings transplanted last weekend. Now grow little babies grow!