trigger updateFields on Concept_Email__c (before update)
{
    //Fetch all the Parent Ids in this Set. Set is used for having unique values.
	Set<Id> parentIds=new Set<Id>();
	
	//Iterate the Concept_Email__c records.
	//Add the Parent Ids into the parentIds Set.
	for(Concept_Email__c emailRecord: Trigger.New)
	{
		//The FieldAPINameForParent__c should be the API Name of the field on Concept_Email__c object linking to the Parent Concept_Master_Contact__c
		//(Important: Please change FieldAPINameForParent__c accordingly.)
		parentIds.(emailRecord.FieldAPINameForParent__c);
	}
    
    //This list will hold the parent Object records.
    List<Concept_Master_Contact__c> conmast = new List<Concept_Master_Contact__c>();
	conmast=[SELECT Unique_Person_ID__c FROM Concept_Master_Contact__c WHERE Id IN: parentIds];
	
    //This list will hold the Concept_Email__c records to be updated.
    List<Concept_Email__c> conemail=new List<Concept_Email__c>();
    
	//Iterate the Concept_Email__c records
    for(Concept_Email__c obj: trigger.new)
	{
		//Iterate the Parent records
		for(Concept_Master_Contact__c parentRecord: conmast)
		{
			//Match if the obj(Concept_Email__c) record and the parentRecord(Concept_Master_Contact__c) Id matches.
			//If yes, then check if the Unique_Person_ID__c field on obj(Concept_Email__c) is not equal to Unique_Person_ID__c field on parentRecord(Concept_Master_Contact__c)
            //(Important: Please change FieldAPINameForParent__c accordingly.)
			if(obj.FieldAPINameForParent__c==parentRecord.Id && obj.Unique_Person_ID__c != parentRecord.Unique_Person_ID__c)
			{
				//Populate the value from parentRecord(Concept_Master_Contact__c) to obj(Concept_Email__c) record.
				obj.Unique_Person_ID__c = parentRecord.Unique_Person_ID__c;
			}
		}
    }
}