Feeding Majin Buu at his house will make him bear children. You gain access to this area after making some progress in the story. When he has six children, he will give you a Distorted Time Egg, which is needed for the secret ending and to reach 100% completion for the story. Feeding his children can also provide some nice rewards, such as new clothing and Super Souls. Feed a child multiple times for him or her to change colors and unlock an achievement. You get food from collecting Blue Orbs in Canton City. Collect some Blue Orbs, then complete one fight for everything to respawn. Repeat this as many times as desired.