Signposts were added to the game around May 1, 2006 as a slightly more permanent and noticeable form of writing messages at a location. Once you have two pieces of driftwood in your inventory, a button appears that allows you to construct a signpost and type its text. The signpost will appear as a small icon on the jungle map and will be readable by any player on the same square with sufficient language skills.
Signposts cannot be defaced or vandalized, but they can be destroyed by a determined attacker. Any melee weapon should work. In theory (although there's no guarantee) you could probably punch a sign until it fell over.