The Family Farm is the home of Arlo and his family, as well as the place where he and his siblings, Buck and Libby were born.
Henry and Ida had lived and worked on the farm on their own until their three eggs hatched and Buck, Libby and Arlo were born. The siblings were taught how to work and care on the farm by their parents; and not long after the silo was built they started the tradition to place their marks on it once they have complete their tasks and have earned their place on the farm. Since Henry and Ida were the first to work on the farm they were the first to places their marks on it, Buck and Libby soon followed while young Arlo struggled.
When the corn in the silo was being eaten by a "critter" Henry used the opportunity to help Arlo earn his mark on the silo, however Arlo was hesitant to hurt the cave boy and let him go. When Arlo and Henry went after him Henry restored Arlo that he can find his way home by following the river, but then a powerful storm appeared. Henry was able to get Arlo to safety, while he got swept away by the raging river. Winter was drawing in fast and Ida was worried that they wouldn't have enough food to survive it without her late husband's help, but Arlo managed to reassure her as he told his mother that he'll do whatever he can to help. Arlo later spotted the critter from earlier and went after him, which got him swept away from the farm by the river.
Following his father's directions of following the river and heading towards Clawtooth Mountain, Arlo was able to find his way home and was reunited with his family. Since he had "token care" of the critter Arlo was finally able to place his mark right next to Henry's.