Flattening your image is another way to decrease the size of your image file. It is similar to merging layers in the sense that the individual layer information is combined into one layer, however, when you flatten your image any invisible (hidden) layers are thrown away, and any transparent areas in the image layers are discarded as well. A flattened image will consist of only one layer and have a significantly smaller size than the same image as a PSD or TIFF file with multiple layers. 

Often images are flattened to convert them to a JPEG, GIF, or other format that does not support the preservation of layers.

To flatten an image, make sure that all the layers that you wish to contribute to the image are visible. Next, click the small arrow in the upper right of the layers palette to reveal the layer menu. From the layer menu, choose Flatten Image.

You will see the following alert prompt appear on your screen.

Clicking the OK button will discard any hidden layers and flatten the image. Clicking the cancel button will abandon the flattening process.