for (variable : list_or_set) { code_block }

//Initializing the custom object records list to store the Invoice Records created today
List<apex_invoice__c> PaidInvoiceNumberList = new List<apex_invoice__c>();

//SOQL query which will fetch the invoice records which has been created today 
PaidInvoiceNumberList = [SELECT Id,Name, APEX_Status__c FROM APEX_Invoice__c  WHERE CreatedDate = today]; 

//List to store the Invoice Number of Paid invoices
List<string> InvoiceNumberList = new List<string>();

//This loop will iterate on the List PaidInvoiceNumberList and will process the each record
for (APEX_Invoice__c objInvoice: PaidInvoiceNumberList) {
	//Condition to check the current record in context values 
	if (objInvoice.APEX_Status__c == 'Paid') {
		//current record on which loop is iterating 
		System.debug('Value of Current Record on which Loop is iterating is '+objInvoice);
		//if Status value is paid then it will the invoice number into List of String
		InvoiceNumberList.add(objInvoice.Name);
	}
}
System.debug('Value of InvoiceNumberList '+InvoiceNumberList);