An action is a series of tasks that you play back on a single file or a batch of files—menu commands, panel options, tool actions, and so on. For example, you can create an action that changes the size of an image, applies an effect to the image, and then saves the file in the desired format.
Actions can include steps that let you perform tasks that cannot be recorded (for example, using a painting tool). Actions can also include modal controls that let you enter values in a dialog box while playing an action.
In Photoshop, actions are the basis for droplets, which are small applications that automatically process all files that are dragged onto their icon.
Photoshop and Illustrator come with predefined actions installed that help you perform common tasks. You can use these actions as is, customize them to meet your needs, or create new actions. Actions are stored in sets to help you organize them.
You can record, edit, customize, and batch-process actions, and you can manage groups of actions by working with action sets.
Source: Adobe Help Source