In plain words
In the Purāṇas Vyāsa made known all the stories of how He, though He created the world, Himself came down from time to time in human form. Tamo-guṇa, rajo-guṇa and sattva-guṇa, the three together, became the human shape. Through the sāttvika guṇa came the devas, through the rājasa guṇas, human beings. Through the tamo-guṇas the dānavas took shape. Human beings have greater cleverness of intellect. Keen attainment of Jñāna comes to them. They have the chance to know their own divinity.
A story about the secret of creation: the devas went to Īśvara and said, "Lord! If human beings and dānavas grasp the secret of creation, it will harm Your divinity, so we will keep this Jñāna in the sky itself." So no one recognised it; when human beings prayed in anguish, it was hidden in the water. At last Īśvara, appearing before them for their tapas, placed it in the lotus of every heart. There, within the five elements, the five prāṇas and the five kośas, that principle dwells. You must bring it into experience through sādhana. People keep their valuables safe inside the house. In the same way, you must search for the principle of the Ātma only within. Those in the position of servants do not find it; you must make yourself master of the house; only then are you worthy of the gem of the Ātma. So long as the vision looks outward, you are nothing but slaves. The very moment inner vision is won, the Light shines forth in the form of the Ātma within the cave and becomes manifest. When clouds such as Ahaṅkāra, envy and desires drift across that sky, the Light is hidden.
Guru Pūrṇimā means the day on which the Manas, turned inward, shines free of saṅkalpa.
If you carry out sādhanas with discipline and steadfastness, keeping to set times, the light of the Ātma will shine. But study brings no result. In truth, who are the makers of unrest? The educated alone are its root cause. Criticising everything, talking too much, multiplying argument and counter-argument: through these, Śānti goes far away. You must join spiritual learning to your studies. You must carry on with worldly life keeping the spiritual as your aim. That is, it is not going to the temple and promising God, 'If I make a profit in business, I will give You half'; you must keep on reducing desires through dhyāna, and hold the object of your dhyāna itself as your aim. 'Niveditā', it is said, asked Vivekānanda for advice, telling him that she could not attain one-pointedness in dhyāna. In dhyāna a person called Margaret Noble comes in between. (That was her former name.) 'That personality must be rooted out,' and, 'It must be made an offering to divinity,' he taught.
Yielding to nothing, neither weariness nor sloth nor despair, to meditate on God in human form, to become people of good qualities, to cultivate a vision of Prema towards all beings and to experience Ānanda: this alone is the message of Guru Pūrṇimā.