If a message has this hidden tag, it would be invisible to all members unless they have a certain role permission to see it. This feature could have many uses, for example, let's say you have dyno setup to block certain words, typically what it would do is just delete them. Instead what it could do using this feature is put a hidden message tag on it, and a moderator can then determine if the message is allowed or not by manually removing the tag.
Another use this feature could have is held for review images, you could set a bot up so when a message that includes an image is sent, it can automatically put the hidden tag on which could then be removed by a moderator once it is approved. This way a server does not have to run the risk of members sending inappropriate images, while still allowing the ability of sending images.