Bleaching is a general term given to the loss of an anemone’s zooxanthellae; so named due to the washing out of colour with the most severe cases the anemone appearing white. The term bleaching is accepted as the mass expulsion of zooxanthellae. This can happen for a number of reasons - mainly due to some sort of environmental stress. This covers excessive temperature changes, insufficient lighting, excessive lighting, excessive salinity change, etc. Physical stress can also cause bleaching.
The most commonly observed bleached anemone is Heteractis crispa. These animals arrive in your LFS an aesthetic white, but this is never their true colour; they should be a fawn/tan colour, or less commonly pink, green or silver/grey.
A supply of energy/nutrition for an anemone comes from the sugars made in the process of photosynthesis by the zooxanthellae. It doesn't take much to work out that if there are no zooxanthellae, there is a reduction in food. Where there are no other nutrition/energy routes, the anemone slowly starves to death, but in an attempt to survive they absorb their own mass, and hence the animal shrinks. One of the most striking example of this is the consumption of an anemones own tentacles. In bleached animals, you often find unusually short tentacles when compared to healthy specimens.
Bleaching can be reversed, but it takes a long time and commitment by the aquarist. In the first instance, excellent water quality should be provided. Lighting should be of the optimal strength, although initially, it would be best to acclimate the animal to stronger light over a period of time. Feeding should be regular, say every other day, and should only be in small quantities (10mm cube for an animal of less than 300mm say). Flow should be restricted slightly in case the animal has trouble attaching/feeding.
Over time, if one is lucky, a gradual change in colour should occur. This is the recovery of the zooxanthellae and is a very good sign. The colour change may be uniform, or it may be sporadic across the animal, but eventually, if all conditions are right, the entire animal will become "as new".