The Cannibal Reactor is an Assault Titan Module. It grants the Titan it is mounted on a stackable damage bonus against other Titans only.
The Cannibal Reactor is an extremely specific Titan Module, granting the Titan it is mounted on a damage bonus against other Titans only. Its main rivals are the Antimatter Reactor and the Singular Reactor. If you usually focus on destroying other Titans while inside your Titan, you should ideally mount the Cannibal Reactor on your Titan, as it grants a higher damage bonus than both other modules. If you don't really care who you are going against in your Titan, you should ideally mount the Antimatter Reactor, as it has a damage bonus against normal Robots as well. However, if you cannot afford the Antimatter Reactor, you could go for the Singular Reactor instead.
A feature not many commanders notice about Titan Modules are that they can be upgraded. By upgrading your Cannibal Reactor, you will not only burn off extremely huge amounts of Platinum, you will also gain an extremely big damage bonus that when stacked, can give an extremely huge amount of damage against enemy Titans. It is recommended to invest in other Titan Modules first before upgrading your current Titan Module, especially if they're in offers, as if you are hunting for many different effects for your Titan, you will be able to get minor amounts of many different effects which will usually prove to be more useful than one effect that has a big impact.