In plain words
“At last we have found a great hero who knows archery like this!” they rejoiced. Learning of this, Bhīṣma appointed Droṇācārya guru to the Kauravas and the Pāṇḍavas.
Once Ekalavya came to Droṇa and begged him to accept him too as a disciple and teach him archery. But Droṇa would not agree. “I teach only princes who are brave and bold; I will not teach a forest-dweller like you who hunts animals,” he said. Even so, Ekalavya bowed to Droṇa and settled it in his mind: “He is my guru.” Though the guru gave him no instruction at all, when he went home he made an image of the guru out of clay, worshipped it and, remembering the guru, learnt archery. In his room there was nothing but the image of his guru. Because he practised in this way, his mind fixed on his guru, he won great mastery in archery.
One day Droṇācārya went to the forest to hunt, taking his disciples with him. While they were all seated in one place, a hunting dog began to bark. Just then an arrow came from somewhere and lodged in the dog's mouth. At once another arrow came and lodged in that arrow. Arjuna wondered who the archer could be with such mastery of shooting at a sound, and looking for him he found Ekalavya. “Who is your guru?” Arjuna asked. Ekalavya looked at Droṇācārya, bowed, and said, “He is my guru.”
Arjuna looked at Droṇācārya angrily: “You gave your word that you would teach shooting at a sound to no one but me. Trusting your word, I have stood bold until now,” he said. Then Droṇa: “Child! I am not his guru; I did not teach him this skill,” he said, and Ekalavya: “What he says is true. But I pictured his form in my mind and learnt this in the spirit of a disciple toward him,” he said.