In plain words
From ancient times many great souls have worshipped God with Bhakti and śraddhā of many kinds, and that worship is itself a sign of gratitude. That is what came to be called Bhakti. It has spread through the world along nine paths.
The nine forms of Bhakti
Śravaṇaṃ kīrtanaṃ viṣṇoḥ smaraṇaṃ pādasevanam arcanaṃ vandanaṃ dāsyaṃ sakhyam ātmanivedanam
Śravaṇam. The sacred one who, by hearing alone, came to believe in the Divine as the principle of the Ātma and merged in the Divine, was the emperor of Bhārata, King Parīkṣit. At all times and in every condition, to proclaim to all life that humanness is a gift of grace, ‘Jantūnāṃ narajanma durlabham’, that a human birth is the most exalted of all, Nārada sang without ceasing, and through kīrtanam he paid his gratitude. Prahlāda, who as a boy received the divine light and was worthy of grace, showed his gratitude to God without ceasing, at all times and in every state, by remembering the Name alone, ‘Oṃ Hari, Oṃ Hari’. Others again offered service to the radiant Divine through pādasevā and so paid their gratitude. Akrūra offered his gratitude through vandanam. King Pṛthu offered his gratitude through arcanam. Hanumān, too, made known his gratitude to Rāmacandra through service. Arjuna offered his gratitude to Kṛṣṇa through friendship. Emperor Bali offered his gratitude to God through the offering of himself. All the sādhanas of our day, too, are given by God. To offer gratitude for the good, the happiness, the Śānti and the joy that God has granted us is the chief duty of the devotee; and this very thing has been spread through the world under the name of Bhakti. At certain times, even though they have divine feelings, they suffer a fall now and then.