What is the poem about royalty marrying their daughters and hoping to come back but not coming back?

In the Book of Songs, there is a poem Yan Yan written by Wei Jun to send his sister to get married: Yan Yan flies in the air, but her feathers are different. My sister married in the country today. I can't see the shadow, and my eyes are like rain. Swallows are flying around in the sky. My sister is far away today, so it's not too long to see him off. He can't see anything, and his face is covered with tears. Swallows are flying in the sky with low voices. Sister is far away in the south today. Send him to the south. It's sad that there is no shadow at all. The integrity of the second sister is really long when you think about it carefully. Gentle and obedient, cautious and kind. Think of the monarch first so that I can be ignorant.