If a logged in user closes their browser, their session is destroyed and they will have to log in again. You can set things up so that if a user checks a remember me checkbox, they will remain logged in even if the browser is closed. This basically turns the login cookie from a session-only cookie to a persistence cookie.

To enable this feature go to Realm Settings left menu item and click on the Login tab and turn on the Remember Me switch:

Once you save this setting, a remember me checkbox will be displayed on the realm’s login page.