Then he showed me Joshua the high priest standing before the angel of the Lord and Satan standing at his right side to accuse him.
This vision is of a sort of trial in heaven with Joshua standing accused. The Joshua that is mentioned is not the one that led Israel into the promised land but another man with the name that was the high priest during Zechariah's time. It is also interesting to note that the name Joshua means "Jehovah saves". Normally, when someone is standing on the right side it is to help the person (that is where we get the term "right-hand man") but that is not the case here. This is typical of the way that the devil operates in that he usually twists things around to where they are not what they appear. That is why he is called the father of lies.