Message Blocks is a feature on Voxelmanip Classic that allows you to embed messages inside of a block, that will be shown when a player hovers over the block.
It originates from the MCLawl family of custom servers, where a message can be attached to a block that gets sent privately into chat when the player punches the block.
You create a message block by using the
/mb command followed by the message. Then punch the block you want to put the message inside. It will now show up as the block's "infotext", on the player's HUD.