Sanātana dharma or Hinduism(as the word is used synonymously) is the religion followed by Hindus, living in Bhārata(India), Nepāla(Nepal), Śrī Laṇkā(Sri Lanka), Pākistān(Pakistan) etc countries. Sanātana is also the name of one of the four kumāras who cursed the Dvārapālakas(Gatekeepers) of Vaikuṅṭha loka(The abode of Lord Vişṇu, a place of eternal joy, goal of human).
Before understanding the meaning of Sanātana, one must know the meaning of religion. The word religion for Abrahmic religions the belief in and worship of a superhuman power or powers, especially a God or gods. Everyone thinks that Hinduism or Sanātana is polytheistic religion, but it's wrong. When Foreigners invaded the Indian subcontinent, they related the religion with their religion. There is only one God in Hinduism, yes, there is no other God in Hinduism.
God is the entity at which a human has to surrender, but in Sanātana, not all Gods that one sees need to he surrendered at.
In Sanātana, Religion or Dharma is not following a messiah and a holy book, but the duties that every human has to fulfill to sustain a life, approximately, Religion is a way of life.
All the readers might have heard the term Karma. Karma means action, that's it. The correct term for what one understands by the term Karma, is Karmaphala, which means 'fruits of action'.
It is obvious for science students, that every action has an equal and opposite reaction, equal in magnitude, and opposite in direction to the action. Likewise, whatever one does gives out a reaction. It is simple to see, but very complex in nature.
Overall, Sanātana dharma is a religion, in a Brahmic sense, one book can't describe the whole religion. In further chapters, deities, karmaphala, universe formation, evolution and everything else would be studied.