It means they are #wiki posts.
We once decided to use the 
symbol to make them more distinguishable from “normal” topics, but lately we haven’t always strictly stuck to that convention. As you can see there are 100 Topics tagged with #wiki.
PS: Once discourse (forum software) supports emojis in tag names I hope we can use the 
icon in the #wiki tag instead of the in the title.
We are using flag-emojis already to indicate non-english topics, although without official support it still has some downsides.