When creating an Audience in Localytics, you have the option to target behavioral criteria where users have been active in the last "X" amount of days. When targeting users during the last 1 day (as seen in the screenshot below), you will be targeting users who were seen as active one day from the current date, as well as users who have been active during the current day.
Essentially this means a user has had a session yesterday and/or today.