Every contact has a score. Rules under Settings → Contact scoring add or remove points when someone opens, clicks, subscribes, submits a form, unsubscribes, bounces or complains.
Daily caps
A cap stops one busy day from inflating a score, for example at most three points per day from opens.
Decay
Points lose half their weight after the half-life you choose (60 days by default), so the score reflects recent interest. Scores are recalculated every night; press Recompute now after changing the settings.
Using scores
Build segments such as "score of 50 or more", branch automations on them, or sort contacts to find your biggest fans.