In plain words
King Drupada arranged a svayaṃvara for his daughter. Just as King Janaka had proclaimed that he would give Sītā in marriage to the hero who could string Śiva's bow, so King Drupada proclaimed that he would give his daughter in marriage to the hero who struck the fish target. What is the fish target? Below, there is water. Up above, a fish keeps turning. Looking at its shadow in the water, you must strike the fish turning above. This is not so easy; it is not a feat that ordinary people can manage. Even so, kings, princes and young men came to the svayaṃvara in their thousands, hoping to wed Drupada's princess. The five Pāṇḍavas, who had come in disguise, were also in that assembly. Great heroes and warriors, with the courage to believe they could win at that svayaṃvara, came forward and tried to strike the fish target. But no one could do it. At the very last, the whole assembly stood stunned. Then Kṛṣṇa turned His gaze towards the Pāṇḍavas and smiled. The Pāṇḍavas whispered among themselves and roused Arjuna. Arjuna, calm by nature and unhurried, rose and, smiling, came up to the fish target with dignified steps. Taking up the bow and looking into the water, he loosed an arrow upwards and struck the target with ease. Seeing this sight, the people cheered with joy. Draupadī placed the garland of flowers her father had given around Arjuna's neck.