An inactive action is an action you include when you are planning a project because you thought of it at the time but you don't need to act on it now. You probably don't want to see it cluttering your actions list either.
When you review your projects (especially when the project icon is red indicating that there are no more current actions for that project), you can change the (inactive) action status to make it current.
If the automatic sequencing for that project is set, the application will set it automatically to the Do ASAP status once the previous action in the project has been set to Done. The automatic sequencing is described in ThinkingRock help under section 05.Review, Automatic Sequencing.
You can also specify a Start Date when entering an inactive action (the field needs to be first enabled in the Preferences/Options). When the start date is reached, TR will set the action status automatically to Do ASAP status.