Short answers, pulled from the story.
The term rta translates to that which has moved in a fitting manner and represents universal natural order. It derives from Proto-Indo-Iranian truth and connects to Proto-Indo-European concepts of being properly joined, right, or true.
Hermann Oldenberg surmised in 1894 that the concept of rta originally arose during the Indo-Aryan period. He argued this emergence came from consideration of the natural order of the world as having causal necessity.
The term rta appears as many as 390 times in the Rigveda. This frequency characterizes it as the one concept which pervades the whole of Rgvedic thought.
Varuna attained the position of universal Power par excellence maintaining rta. He separated and established heaven and earth while spreading them out as firmaments.
Dharma grew to overshadow rta in prominence in later Vedic literature. It represents aspect of universal Order concerning mundane natural, religious, social and moral spheres.