Action Rule is the most frequently used Rule in InfoPath forms where you want to perform any action on control of the form or on form load or want to submit the data. Also, on firing the action, you can show the Validation Error message.
InfoPath forms are quite user-friendly to work with. They come with lots of default setting for the user to work easily. One of those functionalities is to set Rules. Rules are set of one or more Actions, which are executed when an event gets to fire.