The Universe Seed is a seed that can be used to create an entire universe.
In the game, Fu intended to use the Universe Seed to create a new universe, using the battles on Prison Planet to collect energy for it.
The Universe Seed is able to bestow an Ultimate Form upon those who use it's power. During the final battles of the Universal Conflict Saga, Kamioren utilized the power of the Universe Seed, transforming them into their "ultimate form" which granted them a tremendous amount of power. After Goku kills them, Hearts receives the now completed version of the Universe Seed and becomes one with it, transforming into his "Ultimate Godslayer" form.
After Hearts' defeat, the Universe Seed grew into the Universe Tree.