A soul egg.
Prestige is a reset-based system that allows you to gain major, permanent boosts, but restarts your Chickens, Research, Habitats, Silos, Vehicles, and Egg type.
Prestiging grants you Soul Eggs based on your earnings.
Improving Prestiges
There are a few ways to improve the yield of Soul Eggs when you prestige:
Soul Food
Soul Food, an Epic Upgrade, strengthens the ability of Soul Eggs. Each level adds a 1% boost to all Soul Eggs (e.g. a level 4 would have a 14% boost). Soul Food has 140 levels, with a maximum of 150% bonus to egg value.
Prestige Bonus
Prestige Bonus, an Epic Upgrade, gives more Soul Eggs per Prestige. Each level adds 10% more Soul Eggs at restart (e.g. a level 5 would have 50% more Soul Eggs than normal (i.e. 15,000 rather than 10,000)). Prestige Bonus has twenty levels, with a maximum of 200% bonus soul eggs when prestiging.
Egg of Prophecy Bonus
You can increase the potency of soul eggs by 5% compounding with prophecy eggs. For each Prophecy Egg you have, it increases the potency of all soul eggs by 5%. You can get prophecy eggs from certain trophies and most contracts.