As an Ōtsutsuki, you are part of a clan of extraterrestrial parasites that consume energy and genetic information from Chakra Fruits, which are cultivated by the God Trees.
Chakra is a supernatural energy that the Ōtsutsuki possess, allowing them to perform incredible feats such as interstellar flight and concealment from others of their kind. With greater control and skill, they can use chakra to perform various applications.
As a Ōtsutsuki, you possess a seed that has the potential to grow into a god tree. However, you can't access higher abilities like the Rinnegan because you haven't consumed a Chakra Fruit yet. You can cultivate the God Tree to grow a Chakra Fruit, which, when devoured, grants god-like power and immortality. Each Chakra Fruit contains unique genetic material and energy from the planet where it was grown. endowing unique abilities and transforming into an improved version of yourself, ultimately leading you to reach godhood and become divine.
To extend their lives, the Ōtsutsuki have developed the Kāma seal, which overwrites the host DNA, transforming it into an Ōtsutsuki. If the Ōtsutsuki is slain, their soul transfers to the Kāma until they regenerate and reincarnate into the vessel. While the vessel is in control, it gains a fragment of the Ōtsutsuki's power and abilities. However, a suitable recipient is required; otherwise, the vessel will not survive the implantation of the Kāma.
(It is important to note that once the seed is sown, you cannot replant it until it has finished consuming all the energy and genetic material on the planet.)