I disapprove of what you say but i will defend to the death your right to say it what it means?

This quotation means that simply because you disagree with another person's opinion does not mean you should suppress it. It means that every person has a right to say their opinion and to express themselves.

If you didn't defend someone's right to say something because you disagreed with them, it would be violating (in the US anyway) their right of free speech and imposing your opinion by default.