I think that Macbeth trusts the vision and doesn’t because he nows that it’s fact and it’s trying to show him a sin and is symbolising that he has a time limit and there is a meaning to killing the king.Thears a prophecy of some sort.I think that the meaning of the dagger is with something to do with king Duncan and with killing him,because from the start of the story Lady Macbeth and Macbeth him self are trying to get there hands  on the power and money and the killing of the king.