A hook is a mechanism that you can use to run or "register" some logic at initialization time. All questions in this regards, should use "hooks" tag.

Hooks allow developers to implement logic, typically initialization functionality to dynamically subscribe to events. They can also cause custom events to be raised by operations which do not raise events by default or to register event handlers programmatically. The system invokes hooks during initialization before any application requests are serviced.

The Sitecore.Pipelines.Loader.LoadHooks pipeline processor in the initialize core pipeline iterates over the /configuration/sitecore/hooks/hook entries in web.config. Then it creates an object of each specified type and invokes its Initialize method.