Adjustment layers are a feature of Photoshop in which certain things -- contrast, brightness, hue, etc. -- can be adjusted without actually changing the image. You add an "adjustment layer" for the property that you want to change, and the layer can be turned on or off -- effectively undoing the adjustment or redoing it -- just as with any other layer.
You can fake many such effects with regular layers, by making copies of your base layer and applying whatever effect you want to it.
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