I think it means that whoever is saying this doesnt agree with what this man said, but he also belives that if what the other guy said shouldnt be said without defending it.
What the following quote "I may not agree with what you said, but defend to the death your right to say" may signify is that even though somebody might not agree with your opinions/ideologies, you must still stand your ground and not let anybody take your voice away.
The Missouri Compromise was a temporary and relatively unsuccessful compromise that said slavery would be forbidden in the Louisiana Territory but allowed in Missouri. It was one of the many acts that led to the Civil War.
