In few words Photoshop Action is a set of commands that are applied to specific image. In one Photoshop action can be added dozens of different complex or easy activities with Photoshop tools, like layer styles, color correction, filters and others. Whereas a single action is combined many acts, they can save you time. For example, you know what color and light balance you want for your more than 100 photos. Simply create action with all parameters and apply it to all of your photos. That can save a lot of your time. Create Photoshop action is not as difficult as you might imagine.
Landscape Color Action

Action: Soft Honey Effect

Check Full List of Photoshop Actions