The terms Active and Inactive are found in many of the Inspect setup windows, including the Parts, Sub-parts, Locations and Concerns windows. An active part is available for an inspector to use whereas an inactive part is not available to have quality data attached to it.
An Administrator might set up inactive parts for products that have not yet been released. When the product is released, the Administrator simply changes the part from inactive to active with the click of a button. Such preparation helps ensure that new and modified products can be launched without losing a moment of quality data.
Changing an item from active to inactive may be useful when it is no longer being used. If you wish to no longer include it in lists or display it in views but you can't delete it for some reason (eg. it's associated to another item) you can make it inactive.
Changing an active item to inactive does not remove the historical data assigned to the item. It only prevents new data from being entered for it while it is inactive.