Sometimes people want to discuss a sensitive or embarrassing topic, without giving away their identity. For this, the Discourse programmers have provided a feature called “anonymous mode.”
If you click on your avatar at the top right of the forum screen, you will see a number of icons: a banner (for access to your bookmarks, an envelope (for sending a private message), a spy (for entering anonymous mode), and a cog (for editing your profile). If you hover your cursor over these icons, hints will pop up to indicate their function.
When you click on the spy icon, the system puts you in anonymous mode, assigns you a username made up of a long string of random letters and numbers, changes your avatar to a generic Gravatar, and changes the spy icon to the international icon for do not enter: Clicking on the do not enter icon should take you back out of anonymous mode. You may have to click more than once/exercise some patience for this. If all else fails, log out and then log back in using your proper username.