The Cannibal Reactor is an Assault Titan Module. It grants the Titan a stackable damage bonus, but only against other Titans.
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 your focus is on destroying other Titans, the Cannibal Reactor is excellent, as it grants a higher damage bonus than both other modules. On the other hand, for general damage, you the Antimatter Reactor 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 a gain damage bonus that when stacked can provide an extremely large amount of damage against enemy Titans. It is recommended to invest in other Titan Modules first before upgrading your current Titan Module.