I have designed a website for one of my client's and this is my first freelance work in which I have designed the complete front-end look of the website, he is just asking for some documentation, I don't know what this term actually means, my question here is Am I really responsible for documenting the website, which means is that mandatory or optional, please guide me.
Ask the client what they mean - and get concrete answers.
If the website includes some sort of content management system, the client probably wants a document including screen dumps describing how to perform typical tasks. Many clients will not be using the functionality on a daily basis and therefore need instructions; these must be provided by you - either in writing or by them phoning you every time.
Since you probably have not included documentation in your estimate, your choices seem to be:
Annoy the customer by attempting to get extra money for producing documentation
Do it for free as a learning experience and include documentation in your future quotes
If you have no other current work, I would go for option 2. As a new freelancer, you especially need good customer interactions as a part of your individual brand-building.
What "kind" of website? If it is just a static website, with or without a CMS there is nothing to give him other than the template you made and logins if it is something like wordpress. If it is an application perhaps he is wanting source code. But it sounds like a simple HTML or WP site and the client does not know what he is asking for. If you are hosting it perhaps he wants access, if he is hosting it on his server well, he has the access.