This verse describes Mordecai's public appearance after his elevation to a position of power and authority in the Persian empire. Let's break it down:

"And Mordecai went forth robed in the royal apparel...": This indicates that Mordecai was dressed in clothing befitting someone of high rank and status within the Persian court. He was wearing clothes usually reserved for the king or very important officials. This shows he now holds significant power.

"...and wearing a golden crown, and a diadem of fine purple linen...": He is wearing symbols of royalty and authority.
Golden crown: A symbol of royalty and honor.
Diadem of fine purple linen: The diadem was a type of head covering. Purple was a color associated with royalty and high status. Linen was considered a fine fabric.

"...and the people in Susa saw it and rejoiced.": Susa was the capital city of the Persian Empire. When the people of Susa saw Mordecai in this regalia, they rejoiced. This suggests several possible reasons:

Relief and Joy: The people were likely still aware of the decree that had threatened to annihilate the Jews. Mordecai's public display of power symbolized the reversal of their fortunes and the safety of the Jewish people. Their joy was a direct result of their relief from fear.
Mordecai's Popularity: Mordecai was a respected figure, and his rise to power may have been seen as a positive development for the empire.
Sign of Justice: Mordecai's promotion was a sign that justice had been served and the wicked Haman had been defeated.

In summary, this verse depicts Mordecai's public triumph and elevation to power, symbolized by his royal attire. It also emphasizes the relief and joy experienced by the people of Susa, as Mordecai's position of authority represented their salvation from a terrible fate. It shows a complete reversal of fortune, from being threatened with annihilation to seeing one of their own elevated to a position of power and influence.