Trigger.old of Apex Trigger

This variable returns a list of the old version of sObject records. This list is only available in update and delete triggers

    
 trigger ApexTrigger on Opportunity(before update) {
      // Only available in Update and Delete Triggers 
  for (Opportunity oldOpp: Trigger.old) {
  for (Opportunity newOpp: Trigger.new) {
     if (oldOpp.Id == newOpp.Id && oldOpp.Amount != newOpp.Amount)
          newOpp.Amount.addError('Amount cannot be changed');
          // Trigger Exception 
        }
    }
}