Short answers, pulled from the story.
Ṛta is the principle of natural order in Vedic religion, held responsible for the proper functioning of the natural, moral, and sacrificial orders of the universe. Sanskrit scholar Maurice Bloomfield called it "one of the most important religious conceptions of the Rigveda". The term appears approximately 390 times in the Rigveda alone.
Ṛta derives from the Proto-Indo-European root h₂r-tós, meaning "properly joined, right, true", and is a distant cognate of the Latin word ordo and the English word "order". It is also related to the Avestan word aṣ̌a, in which the meaning "truth" is dominant. Scholar Mahony translated it as "that which has moved in a fitting manner".
Dharma was originally understood as a particularized manifestation of Ṛta, representing the aspect of universal order that applies to the natural, social, and moral spheres. Over time Dharma grew to overshadow Ṛta because it proved more useful for framing practical religious and social regulations, and it became the subject of an expansive literary tradition while Ṛta remained largely confined to the Vedas and their commentaries.
Varuna is the deity most closely identified with Ṛta in the Vedic hymns. Scholar James described Varuna as the "universal Power par excellence maintaining Ṛta", celebrated for separating and establishing heaven and earth and ordering the immutable moral law through the sovereignty of Ṛta. Varuna is specifically called the "friend of Ṛta".
Karma emerged in the late Vedic period as a doctrine of moral causation that addressed the problem of suffering within a universe governed by the inherently good principle of Ṛta. It was conceived as an extension of Ṛta, operating with the same absolute efficiency, and placed responsibility for suffering on the individual's own actions rather than on the gods or the cosmic order itself.
Scholars have compared Ṛta to Ma'at in ancient Egyptian religion, to Moira and the Logos in Greek paganism, and to the Dao. Some researchers argue these parallel concepts share a common ancestor in Proto-Indo-European culture, since their names can often be traced to the same root word h₂r-tós.