Engagement Index in civicrm Activities Section
Per the documentation, it's "to gauge their level of interest or potential to be a highly active member or be easily mobilised for direct actions".

If you use a methodology that involves an engagement ladder, you can decide that signing an online petition is an index of 1, showing up in person to a rally is an index of 2, recruiting others to attend is an index of 3, etc.  Then you can use CiviCRM to find users who have taken an action of a certain level or higher.  This is also potentially useful to fundraisers who use moves management/donor journeys.