What is the meaning of Voltaire's famous quotation I do not agree with a word you say but will defend to the death your right to say it?

Voltaire is expressing the essence of freedom of speech. If people are allowed to speak freely, they must be allowed to say not just those things that we agree with or approve of, but also to say things that we disagree with or disapprove of. Otherwise, we would be putting ourselves in the position of having a veto over what other people are allowed to say, and that is not free speech. We would not like other people to tell us what we may or may not say, and therefore, we must be equally tolerant of others.