This refers to the process of weaning plants from the protective atmosphere of the greenhouse and getting them used to the great outdoors. As well as the change in temperature, one of the big differences for 'indoor' plants is getting used to the wind. Plants should be put outside, in their pots during the day and brought back in again at night or if the weather changes drastically. This should be done for 2 weeks and then the plant can be left out or planted. House plants that are recommended for putting outside during summer (orchids or citrus) will also need to be hardened off properly. The price you pay for not hardening off properly (or at all!) is a severe check to growth or possibly death! (of the plant, of course!).