It means that your comment was dead (like this one actually) and not visible by default. If you click on a comments timestamp (like the text that says one hour

It means that your comment was dead (like this one actually) and not visible by default. If you click on a comments timestamp (like the text that says one hour ago) you see more options - one is flag, another is vouch. As I understand it vouching is literal - I hit that button to say that your comment disappearing seems like a mistake