Wiki markup is the code we use on profiles and other pages on the site to do things like format text, link to other pages, etc. Technically you can play around with wiki markup to try things out on any profile, and the history is always there in case you mess things up. But people are often nervous about this.
So, I created Space:Sandbox. It's a free space page that exists just for people to play around with wiki markup, html, or whatever. It's open, so anyone who's signed the honour code can edit it; you don't have to request to be on the trusted list.
Have fun! :)