That's a tricky question. To start off, Hagoromo was born with a very special gift, he could comprehend the inner works of Chakra and other universal elements. It was with this knowledge that he broke down the components of Chakra, spreading them to the world as part of his Ninshu teachings. These teachings and lessons were developed into Ninjutsu over time, and are the component parts of all teachings done for beginning shinobi.
Basically, Hagoromo used it to found "The Shinobi Way". Naruto inherited this ability upon gaining Hagoromo's power. With it, using these same elements, he can utilize techniques to the fullest potential, without even training with them, because he "comprehends" them. Note, comprehend doesn't mean he can explain them to you, he just "knows" them.
The ability is one of Naruto's most fearsome, if gives the talent to master techniques instantly to levels far above what Jonin or Kage-level Shinobi are capable of. It's how he made his multi-nature transformation Rasengans and his Tailed Beast Ball Rasenshuriken. Naruto's comprehension of Chakra allows him to alter their fundamental elements at a base level, he is re-constructing Ninjutsu from the base up and making them much stronger.
