What is Mutating Table Error in Triggers

/*****************************************************************************
WHAT IS MUTATING TABLE ERROR???????????????????????????????????????????
*****************************************************************************/
CREATE TABLE mutate_tables1 ( num    NUMBER );

CREATE TABLE mutate_tables2 ( id NUMBER );

INSERT INTO mutate_tables2 VALUES (NULL);
COMMIT;

CREATE OR REPLACE TRIGGER mutate_Trigger1
AFTER INSERT ON mutate_tables2
FOR EACH ROW
DECLARE
PRAGMA AUTONOMOUS_TRANSACTION;
BEGIN
  UPDATE mutate_tables1
     SET num = ( SELECT MAX(id) FROM mutate_tables2 );
   COMMIT; 
END;
/

INSERT INTO mutate_tables2 VALUES (1);

commit;

select * from mutate_tables2;
select * from mutate_tables1;