Vision is an ability that Haru possess. With it, he can foresee the future. He can foresee it when he sleeps. It is different from a dream and Haru can naturally tell apart a vision from a dream or a nightmare.
According to Haru's mother, the ability of vision that he possesses has been passed down to him from his father when he was 17 years old. His father got it from his father and so on. For several generations, this ability has been passed down only to the sons.
His mother does not know if it is a coincidence but from Haru's paternal side of the family, the first child is always a boy.
Haru's relationship with his father is a strained one. It was because his father lost the ability to foresee the future when it was passed down to Haru.
Due to this, Haru's father did not talk about the ability of Haru and neither did he explain how it works.
Whatever Haru knows about vision at this moment are what his mother told him.
However, since his mother does not possess the ability, there is a limit to which she can guide him.
The strange force that told Haru to speak to Rin is a type of vision that one can possess. For this, sleep does not act as a catalyst to foresee the future and can happen when Haru is conscious. However, at the moment Haru does not know about this.