It is a series of recorded commands (also referred to as a Macro) within Photoshop or a relevant program. The action panel has a bullet list of titled entries with nested "actions" or "commands" that alter the whole image and/or selection. In order to add an action, you have to save it somewhere you can easily locate on your computer [be sure you have unzipped the file itself] and then within Photoshop open up the action panel. Then one clicks the button on the top right of the action pane that has a small down arrow next to three horizontal lines (menu) and select "Load Actions...". You then search for the saved action file and select it. It should now appear on the list of actions within the action pane. Whatever selection you have, right click on the action and select "apply".
Often actions are consisted of many layers and you can frequently alter the layers. It is a powerful tool if you wish to create a series of commands that can be used again or applied to several images. They can be as simple or as complex as one can design. They are easy to create, but one has to recall which ones one has created oneself so one does not accidentally use another's without giving proper credit. Hope this sort of helps.
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