This is an idiom, meaning that when the hare dies, the fox feels grieved. It is sometimes referred as "hu tu zhi bei".

The idiom comes from History of the Song Dynasty. As the story goes, once upon a time, a hare and a fox joined an alliance against their common enemies - the hunters, and vowed to share the weal and woe. Unexpectedly, a group of hunters suddenly came and shot the hare dead with just one arrow, but the fox had a narrow escape. After hunters left, the fox ran to the hare and started weeping. At this time, an elder passed by and was surprised at the scene. He asked the fox why it was crying. Sadly, the fox answered, "The hare and I are just insignificant animals and preys to hunters. We agreed to fight our enemy together for life or for death. Today he was shot dead and I may die like him tomorrow. We are real friends. How can I not bewail his death?" The elder sighed at the words and said, "Yes, you have every reason to grieve for such a friend."

It is used to describe someone grieves over the death or misfortune of the same species.