Question
the back of my package of zenia seeds says to harden off before planting outdoors. what does this mean?

Answer
I'm sorry Ester.  I thought I had answered this question. I apologize for the delay.  Harden off means to allow the seedlings to acclimate to the outdoors.  When grown indoors, seedlings do not get exposure to the elements such as wind, differences in temperature, direct sunlight, rain, hail, etc.  Place the plants, after the last frost, in a shaded area for about 10 days.  This will do the trick.  The plants will get the brightness of the outdoors and not fry and their stems will become sturdy from the motion of air circulation.  Zinnias are frost tender so make sure you put them out AFTER the last frost.  I hope this helps.
Tom