From b2evolution manual

Typically this scheduled task should run often but a person with unread messages should only receive one message every 72 hours.

So this scheduled task will find users who:

• have unread messages created more than 24 hours ago
• and have not been reminded during the last 72 hours

The timestamp of the last unread messages reminder email that is sent to a user is saved into the UserSettings table, and it is visible for admin users in the user profile admin tab.