Skoll and Hati are the two wolves, who respectively pursue the Sun and the Moon, and will eventually catch them during the Ragnarök.
In the Gylfaginning (ch. 12), Gangleri asks Hár about the course of the Sun, which goes quickly, as if he were afraid. Hár explains that he is indeed being chased, and says: "They are two wolves and the one chasing him is called Sköll. He frightens him, and he will catch him. And Hati is called the one who runs ahead of him, and he wants to catch the Moon, and so he will."
Odin gave Sol and Mani (the sun and the moon) two chariots to cross the sky and bring light to the world. But the two gods were very easily distracted. And could not help but stop to contemplate the world. Their action had an impact on the gods but also on the humans because nobody knew when to sleep or stay awake.
Odin tried to threaten them, but to no avail, for the two gods knew that they were the only ones who could accomplish this mission.
Loki found the solution by asking Hati and Sköll (his grandsons) to chase them and if they could catch them, they could devour them.
Sköll chased the sun and Hati chased the moon, and so the natural cycle of day and night returned to normal.
This idea is also found in Hinduism where the demon Rahu pursues the sun and the moon to swallow them. He sometimes catches them but, having been decapitated by Vishnu for drinking amrita, he can only cause brief eclipses.
Skoll and Hati belong to the Managarm, and descend of the wolf Fenrir and the giantess Iarnvidia, Skoll pursues the Sun and Hati pursues the Moon, placed on chariots that follow each other relentlessly. At the end of time (Ragnarök) the two wolves will eat the moon and the sun, the stars will be swallowed by Fenrir.