From D&D Wiki
 Sandbox Shenanigans
So maybe you've been reading up on how to play with tables, or some other feature, and you want to see if you can make it work. That's understandable. However, you can't just go and make a random page in the main namespace to practice your coding. You need a sandbox.
If you're really just trying to see how something works, you can edit the sandbox. There is one drawback to this feature, anyone can edit it, and it gets cleaned out pretty frequently. So, instead you'll want to go to your user page. Your userpage is located under the "User" namespace. For instance, my username is Badger, so my user page is located at User:Badger. For more information on your user page, click here. You can edit this page, and use it as a playground and edit box if you want.
Some people (myself included), want their user page to look nice. Thus, the glory that is the sandbox. Also known as subpages, a sandbox is a page you can make that stays out of the main namespace, which cuts down on clutter, and is permanent so you can take as long as you want to do whatever. To make a sandbox, just type in the url of where you want your sandbox to be. It should be in your namespace, so the URL should look something like "Http://www.dandwiki/wiki/User:YourUsername/SandboxName".
Some people choose to leave their sandboxes numbered (1,2,3, etc) and others like to name them (Forest Avenger, Toothed Devil, etc). It's in your private namespace, so you can name it whatever you want, and you can take as long as you want to finish it. It is still part of the wiki, so if you want to give a link to someone else to get their opinion, you can treat it like any other link.
You can make sub-pages of sandboxes in the same way that you make a sandbox. Just add another slash, and then add on more text. Because computers are marvelous inventions, this can be done infinitely. You could make the following sandbox, for instance: "Http://www.dandwiki/wiki/User:YourUsername/Animalia/Chordata/Mammalia/Carnivora/Mustelidae/Mellivora/capensis/Me(Honey_Badger)". Hopefully by now you've gotten the idea of how sub-pages work!