Saul was still looking to kill David.  David hid far back in a cave.  King Saul just happened to enter the same cave to relieve himself.  David's companions told him to kill Saul.  Instead David secretly cut off a piece of Saul's robe.  After Saul had left the cave, David called after him.  He told Saul that he could have killed him, but instead merely removed a corner of his robe.  King Saul was amazed at the clemency of David.