In plain words
Gu, ru. The guru is one who roots out ignorance. The syllable ‘gu’ is the principle beyond the guṇas. The syllable ‘ru’ is the formless. God alone is without guṇas and without form. So they said, ‘Gurur brahmā gurur viṣṇuḥ gurur devo maheśvaraḥ.’ The real guru is God alone. All these are merely like teachers. When a student enters college there are the maths teacher, the physics teacher, the chemistry teacher. How can one worship so many? There is only one guru. For each subject there is a teacher. All these are ones who teach. The guru must show his aim. The aim means: he must bring one into the principle of the Ātma. That is the aim. Guruhood is nothing but a spotless mind. He who has known himself is the real guru.
Śukra and Droṇa were not real gurus
A small example. In the Kṛta age Śrīman Nārāyaṇa took the Vāmana incarnation and went to ask for a gift, in order to set Emperor Bali on the right path. Emperor Bali's guru was Śukrācārya. Ācārya.. One who has practised. An ācārya is guru to many thousands. Emperor Bali, knowing that Vāmana was Nārāyaṇa, made ready to offer Him everything. He gave his word: ‘I will give.’ Śukrācārya: ‘No, no. You will be badly deceived. The whole kingdom will go to ruin. Do not give him the three steps as a gift at all,’ he said. At that moment he defied him, asking what a guru is beside God: ‘Is there a sin worse than going back on one's word?’ Come what may, even if the guru curses me, it does not matter. ‘I will offer it to God,’ he said. Is he a guru who says no when one is offering to God?
In the Dvāpara age Droṇācārya came as the one guru to teach the Pāṇḍavas every kind of skill. Droṇācārya made a promise. I shall be guru to no one but you. I shall offer all my power to you alone. I shall not do so for another. With that assurance Arjuna swelled with pride. He taught him every skill. As he had said, Droṇa went as guru to no one. Once Ekalavya came: ‘Swāmī! Teach me the art,’ he prayed. ‘I have made a promise to Arjuna. I will be guru to no one but Arjuna,’ he said. Up to that point, all was well. The Pāṇḍavas and the Kauravas went to the forest for an outing. They took dogs along with them. Seeing an animal, a dog barked. At the sound of that barking, four or five arrows came from somewhere. Five struck at one time. Arjuna was greatly amazed. They went searching through the forest: who was the great master of the art who had loosed all those arrows by the sound alone? He is a man of great power. I do not have this power. When others have what one lacks oneself, envy comes. As they searched, there inside a thatched hut stood a hunter, holding an arrow. They saw him. ‘Was it you who loosed those?’ he asked. ‘Yes,’ he said. ‘Who is your guru?’ he asked. He had prayed to Droṇācārya. Since Droṇa did not agree, he took Droṇācārya's very feet as his guru and learned these skills. Without his knowing it, Droṇācārya is my guru,’ he said. Arjuna burned with anger. Thinking, ‘On one side he made me a promise, and then he taught this man so many skills,’ he went home in great anger. He asked Droṇācārya. Droṇācārya spoke nothing but the truth. ‘I am guru to no one. I am not Ekalavya's guru. He came to me and prayed, but I did not accept,’ he said. But Arjuna was furious. There is someone greater than me in this world. Great envy. He resolved that he must somehow wipe out that man's power. No one greater than me must be in this world. So he told Droṇācārya to make a fitting effort for this. Droṇācārya, having no choice, saw his high standing and gave in. He feared, ‘If I go against the king, what will become of me?’ With a selfish mind he hatched a scheme and went to Ekalavya, who lived in the forest: ‘Ekalavya! Who is your guru?’ he asked. ‘You yourself are the guru,’ he said. He said it with great reverence. Though this man was not his guru, in his mind he said, ‘You yourself are my guru.’ Then what will you give as the guru's fee?’ he said. ‘Whatever you ask, that I will give,’ he said. He was ready for anything. Because such is faith in the guru. To one so full of faith the guru did a great betrayal.
Because he gave his right thumb as the gift, his deftness in fitting the arrow was broken; the hunter fell from his wealth of archery, and Pārtha's heartache left him then!