A scheduler serves to perform background tasks at certain defined intervals.
As per Liferay portlet DTD
<!- The scheduler-entry element contains the declarative data of a scheduler. ->
!ELEMENT scheduler-entry (scheduler-description?, scheduler-event-listener-class, trigger)
<!- The scheduler-description value describes a scheduler. ->
!ELEMENT scheduler-description (#PCDATA)
<!- The scheduler-event-listener-class value must be a class that implements com.liferay.portal.kernel.messaging.MessageListener. This class will receive a message at a regular interval specified by the trigger element. ->
!ELEMENT scheduler-event-listener-class (#PCDATA)
<!- The trigger element contains configuration data to indicate when to trigger the class specified in scheduler-event-listener-class. ->
!ELEMENT trigger (cron | simple)