Bhakti in Hinduism is true devotion to the deity and considered as ultimate or highest Dharma (Religion).
What is Religion?
Religion refers as cosmic law and order. It is basically the knowledge, teachings, and practices that raise a human being from a lower state of awareness or consciousness to a higher state of consciousness. In other words, it uplifts a soul from absolute material ignorance to the realization of divine bliss.
This progression is not completed in one birth of the human being. There is a requirement for the person is to take many rebirths, and in any given birth, there is a teaching that corresponds to the state of any given person. The aim of that teaching is not to keep an individual at the same stage of Religion forever. It purifies the heart and mind of the individual so that person can keep moving ahead to receive further teachings of Religion, progress on the path of spiritual refinement.
In Hinduism, there are two kinds of Dharma: apar or lower Religion, and par or higher dharma. The lower Dharma leads to our physical and mental happiness. The higher Dharma is for the true happiness of the soul.
Bhakti is ultimate or higher form of Dharma. It is the culmination of teachings, actions, practices and knowledge for the attainment of supreme Divine Bliss.
Bhakti is the true devotion of the human being (Jeevataman) towards Paramataman by surrendering wholeheartedly. It can lead to the path of liberation from the cycle of rebirth and ultimately help in achieving the contentment of soul by leading on the way of the attainment of its ultimate goal of uniting jeevatman with paramatman.