Hello om-sai-ram, and thank you for your question.
Unfortunately it looks like you've placed this question in the Billing category so I'm not sure what software you're trying to use.
When dealing with a CMS (Content Management System), a hook is typically a way to allow you to add functionality to the software by hooking into an already previously created function or process.
What software are you using, and what exactly are you trying to accomplish? You mention both a theme, and a custom module, and the more specific you can be about exactly what you're trying to get as a end result the better we'll be able to steer you in the right direction.
Please let us know if you have any further questions.
- Jacob