The Donner party along with many other families fled west during the westward expansion. They took the Oregon Trail.
Everyone had reached the Sierra Nevada Mountains where The Donner Party took a different trail than the rest of the party. They thought they were going to arrive in California earlier than everyone else would.
They got stuck in the mountains where it was cold and very little food was still available.
Soon no food was left. They killed their animals then when they ran out of the food they packed to eat.
When they ran out of animals, they ate each other to try and survive. Since they didn't have enough food they all eventually died.