An Automator action performs a single step. You combine actions in your workflow to accomplish a series of steps. Automator comes with a library of diverse actions that you can use to create your own custom automations.
To perform a complex task with multiple steps, actions frequently receive information, or input, from the previous action. An action acts on this input and then passes the result to the following action to use for its own input.
Actions can have fields, pop-up menus, checkboxes, or buttons that let you choose different options that control how the action behaves.
Some actions get input from outside of the workflow. For example, an action may get input from an app, such as a contact, an iTunes playlist, or an item selected in the Finder.
For more information about Automator, see the Automator website.